I'm sure they will come up with some plausible reason like the project having incompatible goals, or embracing a different design architecture than the one they wanted.

Truthfully, these 'prestige' projects are done for reasons other than actually improving the development standards. They probably didn't use RDL because it was more fun to write their own then figure out someone else's thing.

For all we know they may not have even had a real need for a new typechecker. Stripe could have a valued engineer who seemed bored so engineering management gave him or her an intellectualy interesting project to do - even if it wasn't that important to the business.