I think Sea of Thieves on PS5 will pave the way for other Xbox exclusives if it's successful. They are just testing the water now. No pun intended.
I think you're right that they're testing the water, but I also don't think day one multiplatform releases, or even multiplatform for all titles is a guarantee.
At this moment in time, all publishers seem to be readjusting what they're doing.
I would say there's far more been said about Microsoft releasing Sea of Thieves, years after it's run on Xbox began, than Bungie, a first party studio, releasing Marathon on Xbox day one.
To some degree, Microsoft keeping Starfield exclusive (this far) is surely the less likely outcome than Sony putting Marathon on Xbox. By all the arguments that we hear on here - nobody buys games on Xbox, PlayStation has the biggest share of the market, etc. etc. with that in mind, Surely Bungie would be better off putting all their efforts into the PS version.
I'm not saying that any particular outcome is more or less likely but I could see all publishers experimenting over the next couple of years.