Well there's part of the problem. "Should I use Slack or this long list of open source options, all trickily alike?" The open source team (which I would prefer to win) would do much better if their message was "use this one, canonical, excellent option."
You say that like there's not fragmentation on the closed-source side as well, with Slack, Hipchat, Skype, etc. The reason there's competition is because ~"let's only field one perfect option" sounds great if you think you're the one with the perfect product, or you're a user whose needs are exactly like that product's pitch. In reality, user needs are diverse: there's no singular perfect product.
In this particular case though, note the title of the article. Wouldn't it be much more convincing as "A Superior Alternative to Slack (Which Happens to be Open Source)"?
EDIT: You're second point definitely does apply here though.