Slate are trying to cut cost everywhere they can to provide the cheapest barebones EV truck possible. My Volvo EX30 also lacks door speakers and while it's not top tier it's fine tbh. Volvo just put a giant speaker bar across the base of the windshield.

So the equivalent of replacing a 5.1 surround with a beefy soundbar?

Essentially yes. I'm not gonna pretend it's better or even equivalent but it's not terrible. If you want a car with incredible audio there's plenty of options, but some people don't care and just want the cheapest car possible and they'll listen to music through a bluetooth speaker during their commute.