Pretty sure they provide an IRC interface, but almost certainly don't use IRC internally. There's almost no way they could support any of their fancier features using IRC. Reactions etc would be horrible to implement.

Yeah and the IRC interface has been getting worse recently. When they added the shared channels across teams they completely broke being able to '@' a user from the IRC gateway. Support said something along the lines of 'yup and we're not planning on fixing it'.

I'm expecting them to completely turn off the IRC gateway in the next year or two.

Embrace, extend, extinguish. It's the SaaS business model.