I often feel like when you see these kinds of situations, whats really going on is that the fighter is just so much better than his pool of competition that he hasn't really been tested yet. A plan to train every thing is basically a plan to train no thing; all else being equal, the guy more specialized in more essential aspects of the game, who can beat his counterpart in that aspect, will then oblige his counterpart to in turn emphasize more of what his own true strong points are, in order to remain or gain a competitive edge.
To borrow an analogy: if you imagine an elite level opponent has like 700 'defense', while you have a whole bunch of 'moves' at around '500-600', it doesn't matter how many different ones you can employ if its all equally fruitless in the end. If you want to get over that hump you need to invest more of that into fewer of those that will let you break through that ceiling.
As a general rule, a golden hammer beats a magic toolbox in combat sports. You need to be familiar with everything, so you can stall, defend, or avoid everything as necessary, but your proactivity, imposing your will on the opponent, depends on having an A-game.
I agree, and this isn't limited to MMA. It's just more pronounced because there's so much to learn. That's where the whole idea of critical distance comes in. Everyone at the elite level of all combat sports has some kind of specialty, and the specialists in more specialized sports will have the most specific A-games.