Keeping Bethesda exclusive to Microsoft's eco-system both adheres to Phil's views on exclusivity being a "bad thing", and will bring in enough revenue to make the acquisition worthwhile.
First off, Spencer isn't referring to exclusivity on a 'console-to-console' basis. He's simply talking about not limiting access to these games by putting them behind a $500 paywall. That's why Gamepass exists. It's always been about accessibility and affordability.
People saying that MS will miss out on millions of sales without the Sony consumer base are missing the point. Playstation has a bigger console market share than MS, we know this. But Playstation's market share is dwarfed by PC, which is in turn, dwarfed by mobile. Having Gamepass on a console, PC, mobile, and eventually as an app on your TV, gives MS a strong foothold in something like 82% of the entire gaming market. By offering their games on Playstation they are removing that requirement for Sony consumers to buy into Gamepass, and are therefore hindering their own platform. It's not about sales anymore, it's about subscriptions.
THAT is what Phil means when he says "exclusivity is bad". Remove the $500 pay walls, increase accessibility. And it's already paying off massively. Xbox made $15.5B in profit in the last quarter. Twice the Bethesda investment. They don't need Playstation's userbase to make this thing work.