Maybe Java includes the possibility to use Kotlin? At many teams I know its up to the team to write parts of Java libraries or services within Kotlin since it's mostly an implementation detail. But again - I'm not part of Stripe so I can't talk for them.

Yeah, I would be interested in the logic here. I'd assume Kotlin to be a far more productive language for ex-rubyists than Java.

Perhaps it's that they don't believe in the future of Kotlin outside of mobile?

In my experience it's usually JVM at these companies, not specifically Java, and certainly in recent years more and more teams use Kotlin for backend services.