I've always thought a compiler was difficult to write until I wrote my first one. Sure it is nothing when compared to a production-level compiler but definitely no longer think that there's something magical about compilers, even the more advanced ones.
I mean, to be a little flippant, all you need is abstraction and application, and you can compute all computable problems, right?
More to the point: what I was trying to do was to point out the irony of saying "building a garbage collector is easy" and then, only a few sentences later, saying "this code may have subtle bugs." Saying "this code is not fast enough for production" would be one thing, saying "this code is not correct" seems to put lie to the assertion that building it is "easy."
I personally have never built a garbage collector in anger. But I suspect that the problem of building a passable garbage collector (that is, one that somebody else would be willing to use) is quite a bit harder than it is made out to be in this article.
This is a pedagogical technique. Your points are valid, but they also ignore that the author is trying to teach concepts through code. That often require simplification.