To sound (kinda) like a broken record, I'm with Chinderblock's thoughs on the matter in the Master Chief
thread:
This as opposed to Dante (Gotta get a DMC game on Switch before we talk Smash, according to Itsuno) and Hat Kid ("We wish she was in Smash, but nope she ain't" basically), where specific reasons are given. Master Chief's case sounds a lot like a decon, but isn't a good decon not just because of how vague the tweet is (and we're not given a specific reason), but also because Microsoft and 343 could be under NDA.