It is entirely out of the realm of possibility. There is absolutely no way in hell that even ten engineers for a couple of months could replicate all of Slack'

It is entirely out of the realm of possibility. There is absolutely no way in hell that even ten engineers for a couple of months could replicate all of Slack's functionality. Even ten times that much would be a stretch. They could maybe make a decent chat app prototype in that amount of time. If they are very talented, they might even be able to make a decent chat app MVP that would get some actual users, but there is absolutely no way that they could replicate all of Slack's functionality.