The rant linked to appears to misunderstand the problem and the workaround. @arfon admits there's what amounts to a design bug in Linguist, and so to identify ".m" files, you have to identify a different extension as the "primary" and put the real extension into the "alternate" list. That's a hacky workaround, but it would make the pull request work.

The alternative is to fix the design issue. But that's going to be a lot harder and require more than a few days.