That's just stating the claim again, not providing evidence. How many browser extensions have I written? How many emacs extensions? Fwiw I did everything in Emacs for many years.
Regardless of the differences in the core platform, the ecosystem is where the real power is. The number of full-time paid contributors to Chromium/Electron/VSCode is dramatic, and it shows. The reach of the VSCode ecosystem means there are now competitive ports of Magit and other key packages.
Maybe the Emacs system has a little more power than a browser. VSCode disallows extensions from messing with the DOM to ensure stability; Emacs doesn't, which means less stability and more flexibility. But it's not much use in practice, or I haven't seen the use.
Anyway, Javascript and Elisp are both basically low-end lisps, largely written in C/C++, and running on x86 machines. Neither resembles the theoretical Symbolics machines of yore.
Maybe with billions in funding Emacs could start competing with the Chromium/Electron/VSCode teams. But that's not gonna happen. I can't afford to write my whole environment alone, so I go where the people are.