The feature is not HPWC connected as I have a Version 2 HPWC (non wifi, purchased in 2018) and a model 3, and I have the option in my tesla account that is talked about to limit charging to a percentage of powerwall.
The "issue / concern" with backing up HPWCs with powerwalls is, if there is a power outage during the middle of the night, and you have your car(s) set to charge during the middle of the night, the cars will drain the batteries almost immediately. Charging a tesla vehicle with powerwalls is like trying to fill up 2 D batteries using 2 AA batteries.
While tesla has released a new feature to limit how much the cars drain the batteries in the above scenario, its not released to all tesla vehicles, and we have no idea if it will ever be released to older tesla vehicles. Its much "safer" to not backup those loads but have them monitored, so that when you have grid power, the car can be charged from solar + batteries + grid if you wish, but if there is no grid, the car will not charge. Some of us have had a 14-50 or smaller outlet put on the "backup loads" side so that you "could" charge the car if you want to, during an outage, by plugging in there, but you would not have to worry about the (admittedly rare) chance your car could drain your powerwalls as above.
If I was in a protracted outage, I would only want to use enough powerwall power to charge my car enough to drive to a supercharger or something, personally.
The other concern is the size of the load for those chargers. I would personally not consider putting 2 car chargers on the backup load side (I would ask them to specifically exclude them) but would ask for a 14-50 outlet on the backup side so I could plug in "if I wanted to" during an outage. They are still monitored so as I mentioned during regular use, you can still use part (or all if you want to, for some reason) of your powerwall power to make a small dent in your car charging.