I’m not an expert writer either, just a hobbyist who has written a lot.
Dialogue in itself isn’t an info-dump, but where it sounds like you might stray into it is if you have all this dialogue that’s basically explaining how Group B influenced the whole plot of the story. For a plot that the reader has already read, this might be a bit repetitive, although it very much depends on how the information is presented. Info-dumping isn’t only relating to worldbuilding, backstories, and inner and outer monologues. It can also include telling/explaining the plot to the reader.
My suggestion would be that it might be more effective to sprinkle hints through the earlier events in the book, foreshadowing the existence of Group B, so that when they finally arrive at page 400, you can skip all the explanation of how every plot event was influenced by them, leaving just a dramatic reveal of “it was Group B behind X, Y, and Z all along!” If you have the foreshadowing done right, you wouldn’t need a big explanation of Group B’s involvement, because it’ll be hinted at throughout the whole story. Readers should be able to look at the Group B reveal, tie it back to the previous events, and realise in hindsight that the information about Group B’s involvement was there all along. Then they’ll go “Oh, that was cool! It all makes sense now!”, which is an amazing experience for a reader.
I’m also inclined to say that you likely don’t need the sections relating to Group B asking for Group A’s information (the readers presumably already know this information because they learned it when Group A did), or the subsequent sharing of information. Sometimes telling is actually okay, when it’s to avoid repeating something that has already been shown in depth. Telling along the lines of “Group A and Group B shared information with each other” allows the readers to know that all the characters from both groups now have all the information they need, without needing to “show” the conversation of Group A explaining everything to Group B. For the same reason, you want to avoid repeating Group A’s plan, which they’ve been working on since page 100, to Group B. It’s okay to “tell” here and make it clear that everybody is up to speed on the plan, without repeating what the audience already knows.
If you limit the meeting between Group A and Group B to things that are new to the readers, you’ll likely end up with a much shorter section of dialogue, which will probably have a stronger pace as a result. Based on your summary of the 24 pages, my feeling is the most important parts are the coming to terms with the character’s past (others might disagree, but I love some good emotional impact from a character discovering something new about themselves - this gets glossed over far too often in far too many stories), Group A’s suspicions about Group B’s motives, dramatic reveal that Group B were behind Plot Elements X Y Z, Group B asking for help, and potentially the update to the plan now that Group B are involved (depending on how significantly their involvement changes the plan.)
I hope this helps. It sounds like you’ve put a lot of hard work and passion into this project, and 400 pages is a really impressive achievement. So no matter whether you get it published or not, you still created something meaningful. 🙂