Often Tesla brakes are squeaky after getting wet, or sitting in humid conditions.
The steel brake rotors get a light coating of rust, that takes a few stops to rub off.
The brake pads on Tesla are designed not to rub on the rotors to increase travel efficiency. The car relies mostly on regenerative braking to slow the car down to just a few MPH.
You often can eliminate the squeaks with just a couple applications of the brake pedal.
Would be unusual for your brake fluid to be causing the squeaks. It should be flushed out every couple years anyways. Just normal maintenance.