I am sure of my statement.
Let's have a look then.
When a motor is providing regen (i.e. a motor producing power) for there to be power consumed, the power has to be consumed somewhere.
I think we can accept that for power to be consumed it must be consumed.
With a generator, if there is no load, then the fuel consumption is very low, when there is a higher load, the consumption goes up. This is just the reverse.
A generator transforms mechanical energy to electrical. The mechanical energy comes from what people in the industry call the "Prime Mover". In your Generac the prime mover is an ICE engine. In your car the prime mover is the roadway. As soon as you take your foot off the accelerator the road starts to supply mechanical power to the wheels where it gets translated to rotational mechanical energy which is transferred through the gearing to the rotor of the "motor". Exactly the same as with your Generac except for the source of the prime power.
In other situations where motorized braking is used, the power is often fed into a load bank, basically a resistive source that will convert the power into heat. In an EV, the battery is the load bank.
A load bank can only transform electrical energy into heat. But a battery can also transform a portion of it to chemical energy. That's a big difference.
But when a battery is near full, there is no where for the power to go.
There certainly is! It goes into the battery. If all the ions have moved to the anode, however, it can't convert any more of that energy into chemical energy and so that energy goes into heat and, I have been given to understand, with Li+ into mechanical energy. Both of those can damage the battery. Thus you don't want to send more energy into the battery once it is charged. If you have the prime mover (the road) spinning the machine's rotor and if the controller is spinning the field faster than the rotor then power will be generated and sent to the battery. As you don't want to do that you command the controller to NOT supply a field rotating faster than the rotor.
If there is no where for the power to go, then the motor can't translate it back into braking.
If you have a generator connected to a load and open a switch the power demanded of the prime mover will go down. If you disconnect the battery from the machine that will happen too or if you turn the machine off or down by controlling the field you will reduce the power generated, the power absorbed by the battery and the power required of the prime mover. As the prime mover's power is the product of the torque it delivers and the angular velocity the torque it delivers (which is the torque absorbed by the car) goes down and there is less braking effect. That is what is done in these cars.
Also said, an electric motor simply converts electrical power into torque power and torque power into electrical power.
There is no such thing as "torque power". Power is the product of torque and angular velocity.
One side gives to the other. When the battery is full, it can't accept it. Just like if the battery is empty, it can't give any.
Perhaps this misunderstanding is at the heart of the problem. When a battery is really and truly empty, i.e. discharged below the damage point, it can supply no more energy. But you can overcharge a battery quite easily. You may well have done it. A charged battery will continue to absorb energy as long as you supply current to it. You may have forgotten to remove a drill battery, for example, from its charger and come back days later to find the battery warm to the touch. Now it won't have been damaged because the charger, assuming it is properly designed, will limit the current such that it never charges beyond the damage point.
The controller in the car does exactly the same thing! It limits the prime power drawn from the road to a level that, when it is converted to electrical energy, protects the battery from damage while still putting as much of it into the battery as it can safely accept.
When a battery is cold, it can't give or accept it as high as it can when warm. Hence both the battery is cold and the regen warnings.
The controller takes temperature and charge level into account. Just like the one in your toothbrush.
When you depress the accelerator, it isn't send torque commands to the motor, it's just increasing the battery voltage to provide more power.
This is a very naive view of how an EV motor (or any modern "DC" motor) is controlled. The power delivered to an electric motor depends, ultimately, on the magnitude of the currents to the stator windings and the slip (the relative rotational speed of the rotor and the field). Rate of the rotation is set by 6 switches (two per phase) that send current from the battery, into one phase, and returns to the battery through the other two phases. 3 switches are on at any given time and the triplets (three are 3 sets) are cycled through at the desired field rotation rate divided by the number of poles. In addition, the switches are turned on and off at a much higher rate (PWM) to control the magnitude of the field currents. The battery voltage stays the battery voltage - it is not boosted or bucked. Through some hairy math the controller accepts information on speed, rotor position and desired torque and translates that into field, torque, frequency and PWM commands. There are loops within loops. If you think I am trying to convince you that I understand all this get that idea out of your head. It is very complicated. I summarize this by saying the controller sends torque commands to the machine. And indeed it does but that represents a gross simplification.
This is really easy to test with a simple toy motor and a small incandescent light bulb. Turn motor, bulb lights. Turn motor without a light attached, it's a lot easier to turn.
It just occurred to me that perhaps you think the motors in these cars are traditional DC motors. They aren't! The S and X have three phase induction motors (invented by Nikola Tesla which is how the company got its name) and the 3 and Raven now have one switched reluctance permanent magnet motor (synchronous).