Am I mistaken in this? I'd love to have this discussion with people more knowledgeable than me on the topic.
Wildly mistaken, yes.
It doesn't change the core fundamentals. Sony and Nintendo are both very committed to their individual product releases and both have delayed major titles just in the last few years as needed. So there is no basis in reality that GamePass is the thing letting MS delay their games when no one else could.
Meanwhile with GamePass existing Microsoft pushed out Sea of Thieves when it clearly needed more time in the oven that it has since gotten, to much success. Grounded as well, which is much improved from its "early access" addition to the service but was pretty jank upon initial release. Forza Horizon 5 which released with some real technical/back end issues that took quite a while to fix, and Halo Infinite with a pretty dogshit roadmap on where they were going for continued content support. So every cardinal sin of releasing a game too early committed by Microsoft while coming day and date to GP.
Also the only reason Microsoft can "have escape routes in seeking out other partnerships to palliate the missing games." is because they're funding Xbox off other segments of the company that make billions of dollars per month. If Nintendo ran a similar service and responded to delaying the new Mario game out of the calendar year by handing an eight-figure check plus additional fees over to a third party studio they'd blow a gigantic hole in their annual profits.
Beyond that, tell me the game Microsoft is going to seek out that makes up for Starfield not making it out in 2022? Gotham Knights? A game that about 1/10th as many people care about? MS is massively subsidizing GamePass for people who buy larger blocks of time, that will do more to stem subscriber loss than any third party game that is available for this year as a "placeholder".
You're rationalizing bad news for Microsoft as "good news, ackshwuallee!" assuming the trillion dollar company will continue to splash the pot regardless of profit/loss in the VG sector and that them doing so instead of actually making their own games in a timely fashion is a good thing.