this is actually mentioned in the article, by one of the quoted people.
It works out because of loose coupling + many contributors. People will work on the web browser and others will work on the main OS, and there aren’t too many conflicts or places where each team has to coordinate.
It also works because making a basic OS or web browser or programming language is not hard, 90% of the work goes into the final 10% of perfection. You can a) make minor improvements to the base OS, or b) create an entirely new program and a burst of seemingly-rapid development; SerenityOS is a passion project, so many people choose b.
Furthermore, many of the people working on the OS are still doing so, and some of the people working on the web browser may not have joined without it. So maybe it’s slowing down progress on the main OS but not by too much, the main OS is still improving.
Honestly I kind of predict people will start to branch off and work on a Serenity office, and Serenity painter and Serenity DAW and Serenity calendar and Serenity game engine. This is basically what KDE has done.