I don't agree with this. I've trained with too many people who trained multiple arts, and both paths seem to be reasonable. Brain confusion is a part of the learning process. It will probably take longer to learn the first art if it is learned alongside the second, but it won't double the learning time. And in some cases, waiting until the first art is ingrained makes it difficult to get the core concepts of the second art (like transitioning from Shotokan's angles to Aikido's curves), making the second art take considerably longer to integrate.
It's a pretty individual thing. Some folks prefer to dig deep into the first art before adding another. Others do better with the variety of multiple arts.
First, I agree mostly. I would actually take it further. I think the entire idea of confusion is very overstated. Provided that the skills are being applied in some context so that you are building real skill, you will be fine. People learn multiple languages at the same time, multiple jobs at the same time, all kinds of things. They go to college and take classes that are completely disparate and do just fine. Or take classes that are closely related, and also do just fine. Learn multiple sports at the same time. And when there's a physical component, it's even easier.
The most overlooked aspect (which I run into all the time when coaching and developing new managers) is the importance of having a solid framework. A solid foundation can be the framework, which is what we hear a lot. If you have a solid foundation to build the skills on, you will be fine. In this case, the you are bringing a framework with you from previous experience. The key, though, is that one can be provided to you by the school or system. What I mean is, you can learn a style and then build on it as a core. Or you can learn discrete styles all at the same time, but organized for you in a framework. An MMA school is an example of this. You can learn wrestling in high school, then BJJ, then Boxing or Muay Thai. Or you can learn wrestling, BJJ, and Muay Thai all discretely and also connected within the framework of MMA transitions and drills.
Either approach can work really well, but each has strengths and weaknesses. If well rounded skills and integration are the goal, you might actually be better off learning everything all at once. The risk of not doing so is specialization. Damian Maia, a world class athlete by any measure, is never going to be a world class striker or wrestler, for example. I believe that had he integrated his learning earlier, his BJJ might be slightly less advanced, but his other skills would be markedly improved.