My guess he implemented those using Kotlin Coroutines to build much more scalable and more thread safe implementations. Concepts it offers like Structured concurrency are really powerful.

True, Coroutines were very heavily used. It was kind of magical how services which nobody wanted to touch with a 10 ft pole due to their complexity for almost years got rewritten in mere days or weeks at most. Those big microservices were broken down to even smaller microservices.

Kotlin makes it easy to avoid the "billion dollar mistake" i.e. using null.

That has nothing to do with scale.

Quite honestly though, I don’t see null as that big of a deal. I haven’t struggled with it since I began programming 10 years ago.

A big study by Microsoft (IIRC) said that ~70% of bugs came from NPEs.

That is interesting. I'm not against compiler enforced null checking, but it's not the killer feature that would convince me to switch to Kotlin from Java.

Your mileage may vary, but for me it's crucial—I can't use a language in production that doesn't have null safety. I want the compiler to force me to consider edge cases, and in many languages the biggest edge case of all is "that value you thought was there isn't actually there".

Sounds in this case attracted a better programmer.