Based on iFixit's iPhone 12 teardown and thus the most recent version of the module, I think they might be able to get it into the screen without much compromise. An important consideration here is that it's not just about the Face ID bit, it's about the better camera bit. While it was important even earlier, this past year has made videoconferencing and other remote work rocket up in priorities, and Macs suck pretty badly by modern standards in terms of camera quality. It's a big, by definition both very visible and very social feature (because everyone else in the conference can see how good/bad you look). I think that might be a motivation for Apple that Face ID by itself wouldn't have.
However that aside, thickness concerns don't apply a single bit to the iMac or to a standalone (Mac only) module (or even integration in Apple's ultra fancy display), so that objection doesn't cover the spectrum of Macs.
The current thickness of the MacBook lid (my 16") is <3.5mm, and the faceID module is >3.5mm. Given that you need glass and metal in that 3.5mm as well as the faceID module, you're talking about significantly changing the dimensions of the MacBooks to do this with today's faceID module.
I look forward to the day they do get it fit in there though (hoping that the Silicon redesign allows for it)!
However that aside, thickness concerns don't apply a single bit to the iMac or to a standalone (Mac only) module (or even integration in Apple's ultra fancy display), so that objection doesn't cover the spectrum of Macs.