I unfortunately don’t have a different interruption of it. It is odd wording and may be out of date. But the core mechanics are going to still apply.
What I will give you is my experience. It’s just an experience so take it at that, it will vary for other people.
In my experience killing NPC’s and Bosses is the fastest way to fill the meter. I usually do this a lot because I Thrall hunt and farm resources from what I slay.
Though I feel that my purge meter fills slower now then it did before the sunken city was introduced. Pre Sunken City I was able to fill the meter by myself in a week. Now it seems to take a week and a half to two weeks.
I don’t build much anymore because I already have 8 bases on this server. I do regularly have my thralls making reinforcements and then change them back to bars. I’m not sure this adds much to the purge meter but it’s something I do.