Directly, "the thin blue line" expresses the idea that the police are what separates society from chaos.
It doesn't inherently suggest police are justified in acting outside the law themselves, though, of course, various people have suggested this (interestingly, from both a pro-police and anti-police perspective).
It seems obvious to me that the post was using this phrase in the sense of being a thin shield from chaos.
Rowan Atkinson had a sitcom set in a London police station in the 90s called "The Thin Blue Line". Are you under the impression he was dogwhistling about extrajudicial violence?
I'd never heard of the extrajudicial punishment aspect of the phrase (though I had heard the phrase itself) and it didn't show up when I googled, but I'm not American, so maybe there's some cultural differences.
[0]: https://knowyourmeme.com/memes/punisher-skull
All in all, this could just be another instance of the "culture war" inflaming every other minor disagreement with Ted playfully using the phrase and Marcan misinterpreting it. Or it could be Ted slipping up with their politics. From what I know about Marcan and what can be inferred from his post, they do seem like someone the alt-right would persecute.
I had a look, and it seems that Ted Ts'o is American, so I guess we should assume he understands the cultural significance of the phrase (even though I didn't).