Kojima is the furthest thing from a brand loyalist there is, and he's been developing a game only possible through streaming that no longer has Google's backing, where else would he go with it?
I think the curious part here is - are Microsoft going to put out a game that's ONLY playable on xCloud? That would be a new precedent. A secondary point of interest would be whether Microsoft turned the partnership on this streaming game into something bigger with him
But yeah, if Kojima was talking to Stadia about this game, then it makes sense he'd go to Microsoft next (or MS would approach him). People even speculated to that end when we first beheld the Shelf of Phil. If it's a game that's made for a streaming format, then your options aren't really plentiful.