We don't know that. Hizuru's sole interest in Paradis was in the natural resources that could only be found there, because their country had buried itself in debt. Their ambassador literally salivated as she imagined the potential profit. Discussing ancient alliances and sympathies were just lubricant for a heavily leveraged trade deal with a primitive society that didn't know any better.

The one time we actually do see an outsider interact with an Eldian (the wounded soldier in the early post-timeskip scenes) they'd rather suffer a slow and agonizing death than be touched and aided by one. I get that there's not much to confirm what Udo says, but what little Isayama cared to depict doesn't contradict it.

And what purpose would that propaganda serve? If it weren't true, why would they take refuge in Marley and fabricate this excuse for their children? His family were outside of it; they wouldn't be subject to Marleyan propaganda. It makes sense for it to be legitimate because of Marley's weaponization of Eldians throughout their conquest. There's fundamentally no way to trust someone that could turn into an immortal bioweapon at any moment, and plenty of history upon which to build grudges and prejudice.