I had not considered that selective breeding for less propolis producing bees may be a reason why traditionally managed colonies produce less propolis. It is an interesting thought experiment to ponder.

However, one thing that the article didn’t mention is that most managed hives, with removable frames, are made with wood that has been planed/sanded smooth on both sides. This makes everything uniform and easy to paint the outside of the boxes, but also limits where bees can/need to place propolis.

I have seen (can’t find sources at the moment) some beekeepers recommending roughing up the inside walls of the boxes - either with low grit sandpaper or a wire brush - so as to create a more ‘natural’ hive and thus induce the bees to coat the inside of the hive with propolis like they would do in a tree hollow.