I purchased an iPhone 15 a few months ago and ended up making this discovery myself. CarPlay would refuse to launch unless I enabled Siri. I didn't do any of the Siri setup, or anything but the app would hard refuse to launch unless I went and toggled on Siri. Maybe that's different depending on your make/model, or the specific infotainment system in your car, but in my '21 Kia Forte, Siri is a very hard requirement.