Concurency doesn't mean multithreading. The "yield" keyword in Python enables a for of concurrency and Python is infamous for not being multi-threaded (when interpreting Python.)
You must be thinking of something else. Concurrent Ruby doesn't include any functionality that is anything like fibres, lightweight threads, or M:N threading (I work on the project).
https://github.com/ruby-concurrency/concurrent-ruby for those wondering. Rails uses it as of version 5.