MIMO isn't that complex. It's using multiple antennas to take advantage of multipath propagation. [0]
If you want a very simplistic explanation, it's like giving you multiple Ethernet cables to improve throughput. (To gloss over all the details about RF and boil it down to the practical benefit: more bandwidth)
Per the article, not everything with MIMO is advertised as 1x1, 2x2, etc. You frequently find WiFi routers mentioning 1T1R (1x1), or 2T2R (2x2). Or maybe just the Chinese routers I look at.
This article is mostly on about interference, which is definitely a big issue in populated areas. But there are definitely issues with Linux and the drivers used by AP manufacturers. There's a project: Make Wi-Fi fast [1] which aims to address these issues.
They're making good progress, especially in environments with a lot of clients. Just having one 802.11b or g client can really ruin throughput for newer devices due to the timesharing algorithm used by default.
What I'm looking for is e.g. an explanation of how MIMO works, or why explicit timeslots were considered useful for 802.11n but not 11b.