You say "obviously" first party could refer to Sony or Nintendo, but that is 100% wrong. It is obviously only referring to MS because Sony and Nintendo are not first party when Microsoft speak about first party.I'm going to regret weighing in on this but:
That's really not a good analogy. You used "I" twice, right in "this month I ate the most hotdogs I've ever eaten". It's unambiguous. Obviously "first-party" could refer to Microsoft, Sony, or Nintendo.
EDIT: I never, for a moment, thought that Sea of Thieves was the fasting-selling, first-party, new IP from this generation. There are other obvious candidates, like Horizon Zero Dawn.
This is literally one of the dumbest things I have ever seen argued on the internet. It is not ambiguous at all. People are trying to find things to bash Microsoft for, end of story.