I'll admit that I wasn't sure which way this would go, but having now created an account to play Sea of Thieves, I have to say it.
ManaByte
was completely right.
You cannot use the game itself to create a "Sea of Thieves" account or a Microsoft account. When you start the game for the first time, the game requires you to use a phone, laptop, or computer and go to the Microsoft website to create an account. I used a chrome book that had never so much as gone to a Microsoft website before, so it's not a matter of the game or MS recognizing an existing account. Immediately after creating a new Microsoft account, you are required to use the in-game code to connect your new MS account to the service.
This last part is what really matters, though. After you create the Microsoft account and link it to the game, but before you're able to start playing, you're then required to select an Xbox avatar and create a gamertag. This is separate from your Microsoft account. As a new player, you are not able to play Sea of Theives on PS5 without creating an Xbox account.
As Microsoft strategy adjusts from a focus on hardware to a focus on the Xbox platform, they will absolutely count PlayStation users towards claims of "monthly active users on the Xbox platform," and they'll be able to do so because every one of them will be playing their games using an Xbox account.