In some ways Google Apps for Business is a really good example of a sort of "new category" of communication software. You could plugin apps online, rather than have to host it your self.
A bit of googling shows he's called Slack a "brand new category of software" many times before, but I'm not clear what he thinks it is.
How many corporations use G suite for their email? How many roll their own email systems and use thunderbird?
To the average user, email is a proprietary non-federated system filled with tracking.
i heard they were throwing money at integrations with everything, had a bumpy ride scaling (mysql master-master ffs) and had vague plans for AI that'd help automate your org, not sure where that went. But the VC wants exponential or bust, so there you go.