I don't really get the initial point. The browser is, for most developers, a black box. And it should probably stay that way. Ideally, it is an exchangable black box.
TL;DR: to build efficient applications in the browser, you do need to understand how the browser works. Just like understanding how an operating system works will help you design good applications for the OS (especially when it comes to performance).
I think my concern is that more of us should just follow the standards and build a minimal document for the users. Obviously, those of us that need efficiency should know what we are doing. But, just as the vast majority of programmers today have high zero understanding of cache impact on algorithm performance, the vast majority have little understanding how browsers do their thing.
Is it worth knowing for other purposes? Well, I think so. It is worth knowing just to know, if no other reason. And it is also worth knowing in the cases where you are building something highly specialized. However, when I see things like the template tag making it in to webkit, we are no longer treating it as a blackbox where we focus on inputs and outputs. Instead, a growing number are getting to muck around in the internals.
Is this bad? I honestly can not say. However, I do know that from my perspective web apps have not been getting more impressive. Nor do I feel I am missing something from them. Seems if folks want items that are as efficient as you are alluding to, you should be doing a native application.
Still a neat article. I just find that point odd.