much like the orcs in warcraft 3 happily going back and forth, carrying bags of gold between the gold mine and the stronghold, the grunt track is essentially a track where you do boring maintenance work for the company
these engagements last for a few months until the company realizes "oh wait, we can outsource this to our India team and save money"
then the India team gives you something they screwed up and you get to untangle it
the punchline, of course, is that untangling it doesn't bring new tangible value to the company, so they don't care when it comes time for asking for promotions
had the author shown intellectual rigor by demonstrating projects written in a spectrum of programming languages, some Go, C, C++, Python and Ruby, then they could have transferred out to a different team and gotten off the "grunt track."
accordingly, they would have been on projects that weren't dead on arrival and he would have gotten the promotion he felt he deserved.
in this case the system actually worked as designed: somebody who did not bother to apply themselves and demonstrate a wide spectrum of skills (which would make them far more valuable) got put in the grunt track and was kept in the grunt track
by the way, the whole concept of getting things like promotions because you "deserve them" without merit is basically the epitome of white privilege
don't be like this guy
if you work hard and demonstrate that you have a wide range of skills, you will progress in your career, but if you don't, you will stay where you are at