...except I figure that at least in the US (where most of the people who can exert pressure on Linux development are located), the phrase's use in this capacity is actually fairly widely known. (It even takes up a significant part of the Wikipedia page: https://en.wikipedia.org/wiki/Thin_blue_line)

It seems more likely to me that he used it out of a lack of political savvy than out of an excess of it, and that the pressure will now mount and he will be forced to make an apology or resign outright. It's unlikely that activists would take ignorance as an excuse, since they tend to consider ignorance of social justice conflicts to be a moral failing in itself.