This is a great question and asked in a considerate, respectful way, so thank you! I think I can speak for most lefties when I say that it's never an intrusion to ask a question you genuinely want to understand the answer to and only impolite when it also comes with baggage like derision or judgmental preconceptions. But yours didn't carry that baggage at all, and it's appreciated. I am no expert in this, but here is what I would say to answer:

One thing you will quickly learn about communism if you don't know already, is that, particularly in the anticommunist US, any organizing around it quickly faces infiltration, co-option, and plain dismantlement to a ludicrous extent, mostly by the alphabet soup agencies, but also even by private reactionaries. The red scare shit never went away, and so western communist organizing now exists in an extremely hostile environment where it is easy for anticommunists to spread misinformation about what communism is, and that misinformation is largely accepted at face value, as if it's common knowledge. The degree of infiltration and smearing of communist organizing, and the lengths the feds go to to accomplish it is one of those things where it would sound absurd were it not for the open admissions of anti-communist state actors like the CIA outright listing the tactics they have used to undermine and derail communist organizing.

I mention all that just because I think it's necessary context that has to be taken into account here. Because the ACP, despite what it claims to be, is an anti-communist group whose purpose is to undermine class struggle not fight for it. It is funded and run by feds, with it's membership either being feds, aware of and accepting of the feds, or simply naive and duped. To outsiders who have no knowledge of the history of working class organizing in the US, this may just seem like an excuse, like it's just "leftist infighting," where some groups just want to claim that the other ones aren't "true" communists. To be fair, leftist infighting and splintering is a real thing unfortunately. But that is not what is going on here in what I'm telling you about the ACP which really, truly is a wholly reactionary organization that exists for the sake of undermining communism and actual leftist organizing, and it does so through co-option and smearing (among other tactics).

By co-opting, I mostly mean that it tries to scoop up people who are curious about communism and whose politics may be showing signs of leftward radicalizing and then steer them down an anticommunist, reactionary path instead. But by co-opting, I also mean showing up at events (protests, sit-ins, etc) and trying to shout loudest as being "THEE" voices of communism even though all other real communists see the ACP as the charlatans they are. That leads to what I mean by smearing: they try to make it look like the things they espouse are simply what communism is about, but they are lying, as they go on to say and push for the most reactionary bullshit imaginable - like being very pro-Trump and pro-MAGA. This makes outsiders believe that is what communists believe, it gives liberals the excuse to say "look at those right wing commies, horseshoe theory is real!" Well "right wing communist" is an oxymoron, but that is exactly the contradiction the ACP tries to straddle. They use Marxist terminology and claim to advocate for the working class, attempt to drape themselves in radical left symbolism (like Hammer and Sickles, red flags) while adhering to chauvinism, nationalism, and imperialist ideology and dogma. This is why the ACP, the Nazbols, and the Larouchites - who are all roughly the same nebulous ball of shit and lies - aren't real communists and why I say that their surreptitious but actual reason for existing (though they would deny it) is to undermine communism, not advance it.

In this case, the significance of the ACP taking over the r/TankieTheDeprogram subreddit is that that sub is somehow one of the last real Marxist-Leninist subs on reddit. I am not a regular there, though I peek in sometimes, and I don't know how they were able to stick around when most others were banned. But I was glad there was still some foothold, if small one, still on reddit. ACP taking it over is effectively killing that last foothold, which is very much an ACP thing to do.

So, believe it or not, that wall of text is the very brief, quick overview just to explain the role of the ACP. It would take more paragraphs to get into the actual history of the ACP (and I am by no means an expert) not to mention the other groups I mentioned, Nazbols, Larouchites, all of whom have history that's inextricably related, but it would take another several paragraphs to get into exactly what they are. At the very least you have some terms to search for to start with if you're curious about all that. But as always, be very mindful of whose perspective you're reading and what agenda they might possibly have.