Out of curiosity ( because we use pm2 here, and it works well ) why would you doubt them using those solutions?
Probably because it doesn't scale for their needs/fit rest of their system.
Judging from their other infrastructure talks its far more likely they are shipping the application in a container to Apache Aurora, and having the process managed by Mesos.
I can't remember if all their services are now running on Mesos, but I'd bet their newer ones are.