You'll notice a whole bunch of people who regularly post on the_donald and other Trump-related subs with zero previous KiA post history before that thread. That kind of thing, more specifically, is what some of those moderators may have been referring to.

This is an echo chamber creating attitude. It doesn't address users who are prolific in multiple subs. What happens if I post a lot both in t_d and politics? Is it "brigading" if I post in politics along with other t_d users on a hot topic?

What's the cutoff for someone being "part of the community" and "brigading"? Is it 50%, 40%, 30% of posts? Maybe you need to have roughly equal posts in all subs? Am I brigading if I only post once in a while, but stay within the rules of the sub? Furthermore, if I've "crippled" my account by posting too much in one sub, how do I get "posting privileges" for another sub without being called a "brigader"? Should I maybe post a bunch of "I like this!" posts to get my post count high enough for "posting privilege"? Maybe I should contact the mods first for permission?

There is also the problem of "community" subs vs "default" or just general "discussion" subs. There's no reasonable way to define a /r/politics "community" - principally, everyone is interested in discussing politics, there's no clear membership base, its subscribers to active users ratio is huge showing that people come on-and-off to post. Maybe you could define the /r/politics community as the most prolific posters there, as people who predominantly post there, but then most posts in /r/politics are being "brigaded".

The problem with "brigading" is that many people agree that there is "brigading" but they never address the definition problem. This means they all have their own concept of "brigading" in their minds, and if actually pressed for specific examples they will disagree whether a particular thing is "brigading" or not."Brigading" is also a guilt by association handwave. Instead of holding individual users responsible for what they do, you hold their "community" responsible. The more you enforce "brigading" the more people will resist it via the obviously available tools, e.g. throwaway accounts.