Slack, maybe unfortunately, always invites the comparison to IRC because they borrowed a lot of their early UX from IRC with mentions, bots, channels, and /commands but that's pretty much where the comparison ends.
You can have IRC-style chatbots that parse messages and do things but Slack "integrations" are a lot more powerful and "magic" if your mental model of Slack is just fancy shmancy IRC.
Slack bundles basically all the features you'd have to implement out of band with IRC like chat history, search, user statuses, notifications for mentions, formatted content, media content like gifs and videos, reminders, attachments, pastebins.
Then there are things that you can't really do with the IRC protocol like reactions, threads, editing messages, messages with dynamically updating content (our Github bot for example changes the color of PR messages as the status changes), and embedded content like Polls that only take up one message and don't spam the chat with votes, shared history across devices.