Gmail was not the first email program (which is what the OP asked about). Gmail came several decades after email was invented and already in widespread use. You may or may not realize this fact (depending on your age).
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.