Huh interesting idea and quite easy to test by putting the phone in a blanket in the trunk. It's definitely using the car speakers, and when I do eventually get a call through I am heard clearly, but perhaps that initial interaction is using the phone mic.
A lot of car microphones also suck
But that wouldn't explain why it wants you to unlock it
If it’s misunderstanding the command and thinking it needs an unlock twice, but then finally gets it correctly and does it without an unlock.. that actually could be explained by the microphone
From my experience an iPhone SE 3 in the pocket can hear pretty well