Zoom is trying to compete by launching Zoom email, calendar, and group chat. Probably a lot of the headcount increases are due to moving fast and trying to get to market as fast as possible.
That ... does not sound like it would work. Zoom is a success because the founder and the founding team came with decades of video conference experience and really just re-implememted webex/cisco products without any bullshit. What do any of these people know about hosting email?