story
"duplication is becoming a significant performance drain" == Every electorn app bundles its own browser engine (as opposed to a single browser engine for the whole OS, like a shared library), which increases system RAM usage and hinders performance.
"They want to single-instance Electron with their own fork" == MS wants to support electron apps but they want to use a shared browser instance for performance.
They rightly believe that doing so with their own browser engine is a huge pain in the ass.
Might as well just use the real Chromium for all browser needs throughout the OS including electron apps instead of trying to reimplement it.