I just built the desktop app in <5min by checking it out and running:
make prebuild; make linux-app
Substitute linux with win-wpf or mac.
They also provide both linux and windows binaries from their GH releases.It's only for OS X they do not provide a compiled binary, which you can presumably build easily.
Given that, it's quite entitled to say it's "working solely on app stores".
I'm not Mattermost, but if I made a desktop project in the future I probably wouldn't bother with providing OS X binaries for each release either (I'm a bit out of the loop on what the latest is but doesn't it require Apple hardware and/or a provisioned dev cert even for "sideloaded" apps these days? Is it as straightforward to set up in CI as normal builds?)