And it never was an IRC client, there was just a somewhat limited bridge you could use to connect your IRC client into Slack network, but never vice-versa.

Slack is inspired by irc and uses irc's core approach of channels, usernames, messaging, bots, and more.

And how exactly does that make it an IRC client?

It doesn't, nobody said it was. Re-read the comment.

"its no longer an IRC client" definitely implies that it was an IRC client in the past.

Didn't say it was an irc client, but that it's today's (equivalent) of an irc client.

It's kind of laid out like some irc clients, have you used an irc client before?

I'm well aware they are deprecated.

Besides taking my comment literally (I now have clarified), lets look at it from a higher level:

The concept of channels, usernames, private messaging in Slack is almost all directly inherited from IRC. There is some great new functionality on top of this core, IRC inspired experience.