They had IRC gateways that you could use to connect your IRC client into slack, but I don't think their architecture started like the IRC protocol. It appears to have at one time just been a webapp, with databases for storage for messages. Not anything like IRC's protocol/architecture.

https://www.slideshare.net/InfoQ/how-slack-works

It may have subsequently turned into something more like how IRC servers work, but early on it doesn't appear to have been.