That's one of the few comments where someone says that Mercurial is better than Git. Interesting to me because I have used Mercurial and I like it, I now do mostly Git because that's what everyone uses. And I kind of prefer Git now because I grew more experienced with it, but really, that's a wash, they are both great and fundamentally very similar.
I had some coworkers who preferred Mercurial, the argument tends to be that it has a better user interface (not hard to beat Git on that one...), but also because of its immutability. Core Mercurial makes it really hard to change history, and branches are strongly tied to commits instead of being moving bookmarks (I know that Mercurial has bookmarks, but we used branches). Simple and stable.
Now the arguments you give for Mercurial are absorb and evolve, commands designed for rewriting history, something that we felt Mercurial tried so hard to make difficult (for example by hiding them behind extensions). Something I find quite interesting, has Mercurial changed so much during last years? Maybe that's what you mean by "modern" even though they came out at almost the same time.
About GitHub. The simplicity of its bug tracker is supposed to be a feature. Too often, we find ourselves with all sorts of fields that ultimately doesn't serve much purpose. For example, what about priority and severity? I understand priority: sort by priority to pick the ones you should work on, but then, what's the point of severity? If both are correlated, one of them is redundant, if they are not, which one should I pay attention to? And what's the severity of a feature request? And am I allowed to make the cosmetic feature I really want a critical priority?
GitHub has tags and a description, this can already do a lot. It is messy, but real life is messy, and often, in bug trackers with lots of fields and categories, I don't know what to put in there because it doesn't really apply, but I still put something because I have to, contributing to noise, which is also messiness but shifted elsewhere.
This is not a criticism of the Mozilla way, just that there is more than one way of doing things.