Its literally the first thing in the wiki page. "Line" between apposing forces is the "order". ie Mantainers in this case.
The first time I heard the phrase was from a TV show with Mr Bean when I was like 9 or 10.
As another commenter said, I think the article guy is a bit sensitive or took it the wrong way, since "the thin blue line" when talking about maintainers is very much like they are acting as defense to "outside" forces. Whether that is good or bad for Linux, is debated.
You scrolled half a wiki page to the part that fits your narrative.
The metaphor of a thin blue line is that they "the police" are not in the typical sense very large, like an army, but they do keep the order with a thin presence of rule and order. Sounds like what maintainers do in this case.
You don't just trade 1 powerful ceo for another when you get the chance to leave. Ridiculous.