I'm the author of the post and we built the API on Kotlin and some of them are our earliest Kotlin services. We're using Micronaut OpenAPI to generate specs.
I wouldn't say it's hard to do it in Elixir; I did it as an elixir NOOB, junior dev 4 years ago: https://github.com/ityonemo/exaggerate -- if you look at the code history it was completed in about 3 weeks (but there's a reason this isn't on hex.pm; surely a company with many seniors has the bandwidth to do something like this right and share it with the community)
Sure. I'd frame this question of not whether these endeavors are difficult or possible but rather if they're the best way to use our time and effort to serve our customers.
FWIW on the note of OSS, we've made some minor contributions to micronaut and other open-source projects we use. We love OSS and are happy to make contributions and work with the broader community! The community and ecosystem has been incredible for us so far.