What he means is that Mojang is the game creator splash screen on both PS4 and Switch, it at no point mentions Xbox, Switch has an option of logging in with your Microsoft account to earn achievements but in terms of brand recognition Minecraft falls under Mojang which has Xbox Game Studios as the parent company. As for the age of Minecraft having an impact on people wanting to get it on a new system... Minecraft is pretty timeless in what it offers, it's like Tetris, game is old as sin but put it on a new platform with a lick of paint and it will sell regardless of it being a game which first came out in 1984.
Again, what has that got to do with adding value to the Xbox brand? The money and revenue Minecraft makes goes straight to Xbox Game Studios which Mojang falls under, thus increases profits under the Xbox banner. Its not about shoving the Xbox flag in peoples faces, its about making money which we cannot deny, Minecraft is making a ton for them. This will play a apart in the overall value for Xbox as its making money off more platforms than Nintendo and Sony are it seems. The Outer Worlds, Bleeding Edge, Phychonaunts 2, Minecraft, Cuphead and future titles all are and will be making Xbox money which adds value to the brand.