[chii]

Slack is a superset of IRC, except for the protocol not being open.

The UX in slack is not easily replicated in IRC (for example, how does/can global search work in IRC?)

Therefore, it is actually correct to say IRC is objectively worse in those measures. The only measure that IRC beats slack is the open protocol (which, if I'm being honest, not many users care,ala most existing chat services moved to private protocols and haven't lost all their users to IRC or xmpp).