With your typical rack that uses a pin, it should work without any issues. There isn't anything crazy going on from a technical standpoint. You're just adding a sleeve to make something fit better. That said, the pin may not fit.
I'm also not an engineer, nor a lawyer. But I'd bet they claim they wont work, because they can't vet what kind of reducer/sleeve/etc you're using, and it's a lot easier to just say "no doesn't work" than try to cover your ass with a ton of caveats.