According to the article, it's only available for the Mac and Windows desktop apps.
Too bad.
It's perplexing that it's not built using cross-platform web technology. If you're not going to leverage that, why saddle users with the signicant resource and latency impact of an Electron app?
Noooooooooooo :(
This has been my greatest frustration with slack (and with google voice calls, though I will angrily use Chromium if I have to, which I guess is Google's goal)