This is actually quite common in embedded devices and even elsewhere. Every Apple device does this, for example (the Secure Enclave is a completely separate OS running on a separate computer).
If you think about it, most laptops have been doing something like this for decades as well for things like brightness control etc., not with a different CPU but definitely an OS-like thing (i.e. the BIOS, using SMIs etc.)
The idea of the "single OS, single CPU computer" has been a myth for a while now.
God knows if they'll end up scaling back their goals, but the vision isn't "just" a few custom integrations.
At least since they started running Java on SIM cards.
I think we've developed software with "ROI" in mind for so long, that by now most people forgot how it was to use devices and interfaces that were made with passion and by taking your time, experimenting and finding the right way, rather than just rushing through stuff and optimizing everything for money.
I remember Flipper Zero had a ton of doubters early on too, myself included. I think I'm now willing to give them more slack to actually experiment and create something even more ambitious, as they successfully executed it the first time most doubted them.
It's not rare at all.
The reason you don't see those projects is because they don't make it very far. Big projects take a lot of effort and people and most people expect compensation for their effort. You can't compensate them without ROI.
As an open-source project they have some benefit of getting contributors to do some of the work. The hardware still needs ROI to exist. Making those custom parts requires up-front capital, which is going to need ROI to pay back.