I’m not sure there are any Slack features that don’t fit into stock IRC v3. Basically everything they do is already supported or a draft. (Now just everyone has to implement it)
Now, possibly. But unless I'm totally wrong on the timeline, IRCv3 (not even talking about implementations) wasn't even close to that level when Slack started and thus is unlikely to be the base of their product. And even now, I wouldn't include v3 yet when talking about "stock IRC" in this context.