Should RubySingleThreadExecutor be marked to be a SerialExecutorService again?

[meineerde]

In 455203e, the RubySingleThreadExecutor implementation was updated to use the generic RubyThreadPoolExecutor. With that change, the previously included SerialExecutorService module was removed, resulting in RubySingleThreadExecutor#serialized? now returning false.

I'm wondering if that is actually correct, given that we still have exactly one thread which executes a single task at a time which is pop'ed from the queue serially?

Shouldn't the module be included again, or more generally: shouldn't RubyThreadPoolExecutor#serialized? always return true if its max_threads is <= 1?