I agree it's most likely not a true limitation but just stated by them as USAFRet said. You see the same thing in external drive enclosures. That said, if you want to be sure, your best bet is to contact support again but either ask to be elevated to a higher level of support, to speak with someone with actual knowledge vs someone just reading a script, or to ask them to have someone test and verify. After all, a computer OEM should have the parts available to do this, and if they want your business they should be willing to do so. Otherwise, shop elsewhere.
Also, I assume when you say SATA you mean actual SATA and not M.2 SATA, in which case that drive capacity should be whatever size drive it will take (e.g. if 3.5" then 22TB is the largest currently available). Sometimes, you can only use one or the other of two ports, so either the M.2 slot or the SATA port, but even then, you should be able to get >2TB. But you still haven't given any specific models, so it's hard for anyone to give anything more than general advice/info.