How is regenerative breaking not the same as engine breaking? Other than the lack of ICE in the EV, the engine is being used to slow down the vehicle, it has the exact same effect.

Well it’s not the same in a similar fashion of “1 + 1 = 2” is not literally the same as “3 - 1 = 2”. Maybe they accomplish similar goals but downshifting to brake can be done for as many gears as the transmission contains in a stepwise fashion. But without a transmission in a single speed vehicle with regenerative braking it’s more of a “spectrum” of braking and not directly controllable by the driver.

And for the record I believe “engine braking” is a slightly different mechanism than “downshift braking” but could be wrong here - my manual passenger vehicles don’t have explicit “engine brake” controls but a semi truck might.

Downshift Braking is Engine Braking, it's the same thing. And there is absolutely no reason an EV motor could not emulate the exact same behaviour as a stick shift. It's all electronic, you could just emulate the behaviour of a stick shift vehicle there and give the user a joystick with stick shift labels on top. All the motor has to do is to either apply regenerative breaking at a certain efficiency value. The electronics can ramp regenerative breaking from 0% to 100% in any amount of steps you like.