where [it] comes from

You imply it comes from:

The “thin blue line” symbol has been used by the “Blue Lives Matter” movement, which emerged in 2014

But you link to a Wikipedia article that says:

New York police commissioner Richard Enright used the phrase in 1922. In the 1950s, Los Angeles Police Chief Bill Parker often used the term in speeches, and he also lent the phrase to the department-produced television show The Thin Blue Line. Parker used the term “thin blue line” to further reinforce the role of the LAPD. As Parker explained, the thin blue line, representing the LAPD, was the barrier between law and order and social and civil anarchy.

The Oxford English Dictionary records its use in 1962 by The Sunday Times referring to police presence at an anti-nuclear demonstration. The phrase is also documented in a 1965 pamphlet by the Massachusetts government, referring to its state police force, and in even earlier police reports of the NYPD. By the early 1970s, the term had spread to police departments across the United States. Author and police officer Joseph Wambaugh helped to further popularize the phrase with his police novels throughout the 1970s and 1980s.

The term was used for the title of Errol Morris’s 1988 documentary film The Thin Blue Line about the murder of the Dallas Police officer Robert W. Wood.

I have no idea about this guy’s politics, but it’s a pretty well known phrase with a lot of different contexts.