No, heroku doesn't work that way, heroku lets you spin up processes and those processes can be threaded. I use JRuby and get one process that can use all cores.
I think it's safe to say that typical Ruby deployments (as I qualified) are not using JRuby, especially not those on Heroku. Thus, even if the process spawns threads, they will be limited to one core at a time.
Whether JRuby's threads are actually running on multiple cores simultaneously depends on what mode Heroku is running the JVM in.
Whether JRuby's threads are actually running on multiple cores simultaneously depends on what mode Heroku is running the JVM in.