I don't follow this argument.
1. Linux Mint is Linux. People who run it say "I'm running linux." 2. Linux Mint does not run steam. People who run it say "Steam doesn't work on Linux Mint". 3. This means that the linux market is fragmented.
You can't just exclude Mint from linux for some arbitrary reasons. It's linux and it doesn't run steam.
And Mint is hardly alone. It's very much like android, there's a huge number of possible issues developers need to contend with. This doesn't mean it's a failure, it means that the cost of doing business will be higher than with the relatively controlled Windows ecosystem or the even more controlled console market.