Framework is definitely not Linux only (laptops are Windows by default , but they offer a no-os option to install your own). They are however generally supportive of Linux, and it’s possible that they helped make the fingerprint reader firmware work well with Linux, though I have no specific information that this was the case.
FWIW, most ThinkPad fingerprint readers (except a couple of models maybe) work out-of-the-box on Linux. Like the one on my Z13 for instance, I didn’t have to do anything to get it going on Fedora.
Also, take a look at the libfprint compatibility list here, there’s nearly 200 compatible readers listed, so I wouldn’t exactly say the number of readers that work on Linux are “rare”.
shows the same model over and over again
That’s incorrect - check the hardware IDs, they’re different. If it’s the same model they’ll have the same hardware ID.
Your linked thread is also from three years ago - that’s a long time in the Linux world where things change quickly. I could just as easily dig up a thread about x not working on pretty much any topic, so it’s not really evidence of anything.
Edit: I just checked the Gitlab releases page for libfprint. Not only have they added more devices (that aren’t listed on the page that I linked previously), it also proves that support for many new devices have been added over the last three years. So claiming that nothing has changed in these past three years, is being disingenuous towards the good efforts made by these devs.
