That's untrue. As far as handling the complexity of a project is concerned, adding more programmers will not reduce it, only add more as there's more room for miscommunication, confusion and general fuck ups. A project maintained by a single competent developer is far less likely to be mishandled than one maintained by several.
As the saying goes - "Too many cooks spoil the soup".
It's like having more artists work on a single painting. Having more can complete it quicker, yes, but it won't necessarily make it any better, since styles will clash and the overarching design may be compromised in the process.
So how do big enterprises on big projects handle it? If they're competent, they'll employ coding practices that have been developed and refined over the decades, that, when followed, will prevent these kinds of problems from emerging in the first place. An incompetent developer will not employ such practices, and thus their projects will, inevitably, decay. It happens all the time, and it isn't from the lack of workers, just the lack of competence and experience. (As a matter of fact, having more developers could actually have accelerated the decay.)
From having glimpsed some of the codebase (from his streams), I can tell you the way R2CK is structured isn't healthy. Not for the project nor the developer. It doesn't matter how good of a programmer you are, but you can't have the majority of work contained in a single file called "Game.cs" with 20k lines of code and where any change may break it. That's not how you code. Coding is about making abstractions - making complex things simple. What he's done is the opposite. And now, supposedly, he's adding multiplayer on top? As Judy Hopps would say - "Sweet cheese and crackers!" ...
Despite his earnings, I can't say I envy the position fek finds himself in.