I think the claim is just that recasting at all is divisive? I can't say I'm too bothered myself, though Basco's voice is so iconic it may take some getting used to. Recasting Aang is a no-brainer since Eisen isn't an actor anymore and his post-puberty voice is completely different from child Aang's anyway. And Whitman's voice was never quite as iconic as Basco's, and there's probably room to believe it may have changed; plus, they used the opportunity to recast to someone of Indigenous Canadian background, a better cultural fit for the Water Tribe.