This was already touched on earlier, but I wanted to add on a bit:

The idea comes from how Reddit handles it (MultiReddits) but from my experience it's a feature not many people made use of, and it sounds like a pain to have to constantly create and manage new multi-communities to group together duplicate communities. This shouldn't be a task that users have to manually do.

This is a pretty bad or maybe just naive take that IMO doesn't sum things in a productive way upon Multi-Reddits. That is-- 1) it arguably doesn't matter a bit how many people make use of it, as each person's MR is going to be a custom affair, and it works at the individual user level anyway, 2) on the contrary, it's no trouble at all to build your MR's either quickly or painstakingly, and you can spread that effort across weeks, months and even years. In the end, I find MR's fantastically useful as super-custom feeds that you can use to stay focused on a tight range of topics.

Unfortunately, these kinds of half-baked conclusions tend to suggest to me that OP doesn't have a whole lot of familiarity with either platform at this time. That said, there's a lot of interesting ideas in the article, it's just a little disappointing in various places.