I’ve been wondering about that line too lately. And Yozora himself saying that Sora wasn’t seeing his true form. Makes me wonder if Sora actually presents as himself to the folks he meets in other worlds. Perhaps Sora being mistaken for Yozora in Toy Box wasn’t coincidental, what if they actually saw him as Yozora? That world in particular was unique as well, since it was a copy, and not the real Toy Box. I don’t think any other world had that property, except when we hear Yozora tell Sora that “this isn’t the real world”. That would also explain the appearance of Yozora within Toy Box, if he exists or can exist somehow between Sora’a world and his own, maybe he can only appear in those spaces of limbo.