Not convincing IMO.

If forking isn't a safeguard, then nothing is. Developers move on, and not just because the company they work in got bought out and they were told to work on something else (what he suggests in the article). Also because they might get an offer from another company, or they got bored with it, or any other reason. But with an open source project, a company - in fact any company - can hire new people to work on the code, and anybody can volunteer as well.

To put it another way, if you argue that forking isn't a safeguard, then the code isn't the real core asset that antitrust regulators would need to be concerned about. It would be the developers. But developers are people. They can decide not to work on it if they want. So there isn't really much to do about that - antitrust regulators can't tell people "keep working on X, to foster competition". That would be a violation of their basic rights.