Key thing is that it really doesn't go into what else they do. Mike could be doing editing or something as a normal day job and just working under a pseudonym; I have a couple of friends who work in the writing/editing game in Hollywood and they make a solid six figures doing like 20 hours a week; slightly more during crunch time and slightly less hours during mid season. And this isn't even for the "big" budget productions.
I could easily seem them as a whole taking in 10s of thousands if not 100k+ a month in total revenue, and have been for the better part of a decade. Some of them being in the double digit millions, just from that alone, isn't out of the ordinary.
Also, how long are people assuming the various half in the bag/best of the worst events take? They probably film 2-3 hours (8+ for BOTW), spend 2-3 hours editing (lets say 8 for BOTW), and are done with that for the week/month. And they'll get 600k+ views on each video guaranteed. Plenty of free time to do other things, or invest, or whatever they want, to add to their revenue. It might be one of their primary revenue streams along with Patreon stuff, but there's no telling what they are doing money-making wise outside of it.
They have over 600 videos in their youtube channel alone, with (there's definitely some peaks/valleys) roughly 600k - 1m views each on average. That's probably a few million there alone, before anything else. It's very believable.