From their Tweet they showed that 60K players over the course of two months played 450K online co-op games, of that 1.5% of players where doing so on Linux, 4% on Mac and the rest on Windows. That's pretty well in line with expectations and again far above what the Steam Hardware Survey shows as the current Linux market-share (it's lower due to Asia growth).
The online co-op mode is sadly Steam exclusive, likely due to a reliance on Steam APIs, which is becoming increasingly common for GOG versions not to get online play or Linux versions due to Galaxy not being on Linux.
Linux and Mac OS X versions will be made. They will be released after the full PC version so let's say 6 moths from now but don't mark it in your calendar just yet
That's good news, as Action Squad looks really quite fun!