I guess you mean any software company besides Aspyr, or Feral Interactive, etc… ? Seeing as how those companies have been around for a while, and porting to lesser used OSes is their bread n butter, yeah, I can completely see how unprofitable it is.
Edit to add that you seem to be missing the point here. This is about developing on Linux, not for Linux. The numbers are even smaller, just as the number of Windows developers is smaller than their user base. From Epic’s POV it shouldn’t be that big a deal, as they make their money from devs using the engine and releasing successful products. Opening the engine up to more devs, means more potential for future profit. Just a handful of successful devs that might have used another engine completely, could make the whole effort worthwhile.