10k requests/sec is not really the same thing as the C10k problem which is 10k concurrent connections. You might serve much more or less than one request per second per connection, so req/s and conncurrent connections are not really related.

I also wouldn't say OTP was built to overcome the C10k problem. It is a very good solution for lots of concurrent connections (with a suitable OS), but OTP was built to manage concurrency needs in the context of telephone switches. The constraints chosen to meet those needs does work (very) well to meet the challenges of large numbers of concurrent IP connections, but I wouldn't say it was built to overcome those challenges.