I haven't done any research, but the OS (One UI from Samsung, in this case) has a lot of tweaks to make sure everything works seamlessly and smoothly with the larger screen.
I'm just assuming that the de-googled OSes probably don't play nice with the larger screen, especially while opening and closing the phone.
Full disclosure, I have not tried graphene (I have a pixel 8 on the way that I will be flashing), but they do support the pixel fold and pixel 9 pro fold (silly name).
Even knowing that, I do understand their apprehension. GrapheneOS doesn't really add any QOL features that are not found in AOSP; just security/privacy features. So if that commenter's last foldable was only usable to them due to Samsung's QOL features, I presume they wouldn't find GrapheneOS on a foldable any more usable than AOSP on a foldable.