(I know bits of their "origin story" but am in no way familiar with their early product or whether that feature was around from the beginning)
I would say yes, @mentions make a chat product like slack minimally viable. It is a baseline expectation of a modern chat solution to be able to ping others by name.
However, the minimally viable form of the @mention feature is where you stop. Skip all things that are not “the @mentioned user received a notification when I send the message”
If you disagree with me, please explain why. I don’t at all believe that a Slack MVP that is nothing more than a WebRTC chat demo would be a sellable MVP in 2020. MVP and prototyping seems to be very conflated.