Banks and fintech often view their in-house tech as competitive advantages, and they usually don't care a whit about building cred in dev communities (a common reason why projects are OSS in the first place, to drive recruitment or promote a complimentary platform).

A good outcome isn't impossible, but it might be unprecedented

Rich Hickey literally bet his retirement savings on making Clojure, and owns all the IP. It seems unlikely he's just going to let them smother it.

Nubank has presented their tech several times at a number of conferences and I believe they often sponsor those conferences. It's not a traditional bank, like Airbnb is not a traditional travel agency, and Amazon is not a traditional bookstore. This is not your local neighborhood credit union or ancient behemoth running COBOL mainframes.

And telecommunications companies don't see their technology as a competitive advantage?