Why would someone start with a minor platform? Mac is consistently 5-6 percentage of the market and has been for decades. They honestly have never been a player.
You're absolutely right, but consider that most of the people complaining who indicate their computer type are mentioning using Apple computers (perhaps electron works better on Windows? Perhaps people don't name drop Windows as much? Perhaps Windows users have lower expectations?). Also, consider that the supported hardware base for a Mac app is much smaller than a Windows app, which means fewer devices to test on, etc, and probably fewer operating system versions to support too. It should be (but may not be) a lower effort project than supporting Windows, and it will be easier to kill if it doesn't go well ('whoops that was a bad idea, but only 2% of our users used it' -- nevermind if that's 80% of their user base that could run it)
Ever heard of selection bias?
Because mac users have been the most concerned about native look and feel for decades, most windows users probably couldn't tell you what the native look and feel is because even MS don't don't don't follow one.
But they a still an incredibly irrelevant platform when it comes to userbase, if anything make windows top tier and Mac users can have the half assed version. Slack is typically a business application. You will use it if it's required and Mac users just are not that big in a corporate environment.