I think one of the big points of the article is that teacher quality does matter, but it's not the only factor (as you point out) and you have to establish it at the beginning of a teacher's career instead of trying to manage it as they teach. The standardized testing and so forth that's necessary to control teacher performance is too intrusive into the teaching process, but that doesn't mean the existing teachers are all good enough either.

When I went to college, I took notice of what people did when their chosen majors were too difficult or they washed out. I didn't know anyone who washed out of liberal arts. Curiously, I didn't know anyone who washed out of math or sciences either, but I didn't know many math or sciences majors, at least until I was taking high-level-enough classes with them that they hadn't washed out. In the more career-oriented majors, engineering majors washed out and became business majors, and business majors washed out and became education majors.

I'm not saying there aren't any good teachers. There are plenty of good teachers. There are also plenty of bad teachers. Fundamentally, the Finnish approach would take a generation to solve that problem because even if you raised standards to become a teacher now, you have a backlog of teachers of uneven quality and little clear way to sort the good from the bad.