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).
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.
A more charitable interpretation would be “we’re the only line of defense protecting something good and valuable from the outside world, so people should give significant weight to our opinions and decisions”. Which, to be clear, I would still mostly disagree with WRT the police, but it at least doesn’t explicitly endorse corruption.
It's perfectly reasonable to assume he was aware of the implications of his words and chose to use them anyway.
And by the way, so does Wikipedia: https://en.wikipedia.org/wiki/Thin_blue_line doesn't mention this interpretation at all. The closest thing is this sentence, which is really not saying the same thing at all, and at any rate only presenting it as something "critics argue", rather than the settled meaning of the phrase.
> Critics argue that the "thin blue line" represents an "us versus them" mindset that heightens tensions between officers and citizens and negatively influences police-community interactions by setting police apart from society at large.
Well, now you have!