Try using Slack in a browser on Android. It doesn't work. I find that really objectionable. They don't (last time I asked) distribute an APK. Instead they directed me to a pirate APK site, so I don't believe it's for security.
It does work on Firefox for Android, if you enable Request Desktop Site. It's not optimized for small screens, though.
It does work with Firefox on Android (not Firefox Focus), if you check 'desktop mode'. Weird that you should have to fake the user-agent string to access it.
I find it hard to believe they would redirect you to a pirate site. Are you sure it was a pirate site and not just a third party site hosting apks