Come on.
You know perfectly well that almost every creator who joins/rejoins a IP says "I will build on existing stuff and I totally 100% won't forget/retcon/ignore it!".
Like, all three modern Dr Who writers said that. All of them immediately then started forgetting, retconning, changing without even a retcon, or just ignoring stuff! Didn't make them bad writers (well, I mean, okay maybe Moffat and Chibnall are lol, but RTD isn't!)
Further, Metzen probably thinks he is doing that. But building on things doesn't mean you have to kick every ball, does it? You don't have to make every character anyone previous was interested in into a major character, do you? You literally cannot possibly play out every potential plot thread. Even trying to do that would be GRRM-style self-harm and bonkers.