They wrote GPUI as a business decision, to focus on performance, because they knew that that would be a core differentiator to all the other IDEs out there that use Electron for example. That they also liked writing it (as a "labor of love") is incidental.
Didn't they also invent Electron for their Atom editor when working at GitHub? Which then was hijacked by Microsoft and VS Code. Was Electron (Atom Shell) a core differentiator of Atom? Absolutely. Was it a business decision? No, it was a dream to use javascript/html/css to build a desktop editor. I think, similarly, Zed was an experiment to build a modern desktop app without any C++ legacy or OS API wrappers.