According to the little bit of research I did before making my original post, William F. Buckley Jr. is the guy who popularized the phrase "Don't let THEM immanentize the Eschaton!" And, I don't know why it's in the combined 4th and 5th edition of the Principia Discordia, as it's not found in the 1st edition. However, I do believe your interpretation of THEM is quite different from Buckley Jr.'s. I write this because I'm certain that to Buckley Jr., you and I would be part of THEM.
Further, I'm not completely certain about Kerry Wendell Thornley's interpretation of THEM. This is because Thornley served overseas in the military, would have definitely known Buckley Jr.'s phrase and interpretation, and the combined 4th and 5th edition of the Principia Discordia was published well after the end of the Vietnam War. Now, to me, Buckley Jr. was a well educated, well rewarded, chickenhawk mouthpiece of The Establishment (THEM). In the slang of the Vietnam War/Hippie Era, he was a tool of The Man. So, it is entirely possible that the phrase "Don't let THEM immanentize the Eschaton!" was intentionally, culturally appropriated, or in the slang of the Vietnam War/Hippie Era, "liberated" and included in the Principia Discordia. But again, I'm not completely certain that is the case. And, it's also entirely possible that the authors simply put it in the book as an afterthought, and yet another one of their inside jokes. After all, the intended meaning of the phrase was not expanded upon in any way. And, by comparison, the book devotes significantly more space to many just plain silly topics.
But, each of us are likely to interpret the true meanings of things in different ways, and that's not necessarily a bad thing. For what it's worth, I find absolutely nothing wrong with your interpretation of THEM, and believe it to be a positive one. And, I do agree that something has to be done about THEM.
Lastly, I'll end this post with an unsolicited observation. There is a culturally fair, but extremely unpopular way to put an end to Endless War. Just fire up the ol' Selective Service System again, and make it mandatory for everyone upon turning eighteen (18) years of age. And, I do mean everyone. Then, upon coming of age, every young adult would have to serve between around eighteen (18) months to two (2) years of active duty somewhere in some capacity, and then be subject to recalls for about four (4) more years in the event of national emergencies. But, when those national emergencies are over, those recalled go back home. If such a mandatory universal draft were to be enacted, I'm certain Endless War itself would become so unpopular that even the mighty Military-industrial Complex wouldn't be able to keep it going.