(With a bit of DRM needed for video, so those competitors can't be pure open source. But still, it's not going to stop Microsoft.)
The article admits to this, then kind of ignores it.
No it wouldn't, if the mustache-twirling villain's indent was control. Those competitors are following most if not all of Chrome's implementation decisions, and economically they're highly incentivized follow whatever Google does.
They'd likely remain mostly compatible because all browser vendors do try to follow web standards nowadays. But maybe not more than Safari and Chrome are compatible. And it helps maintain Microsoft's veto on web standards (like Firefox has vetoed previous Chrome proposals).
Didn't Microsoft recently throw in the towel with Edge, and switch to Chromium, because they thought it was too much trouble to maintain their own engine?
There are occasional debates on Hacker News about when you should rewrite your codebase. Similar arguments apply here: when should you rewrite someone else's codebase?
They explicitly said they wouldn't fork Chromium.
But the point is, they can fork, so that means Chrome doesn't have full control.
-- Jean-Luc Picard, Captain, USS Enterprise