I'm surprised by what you say, it is not at all my experience. Are you sure you are not over-interpreting what your friend said, or that your friend's experience was not unusual?
1) People at CERN publish papers in "normal" physics journals, which do the usual peer review. Few articles that I've myself per-reviewed were not from my own experiment. There is, of course, also an internal reviewing for each collaboration, but it is to improve the quality and something totally natural and obvious if you want to have a collaboration (by definition, a collaboration is a place where people read each other work and feedback to each others). But it is totally different from "the work is only reviewed by the collaboration".
2) I've worked ~5 years in one experiment, and ~5 years in another, and I did not notice any different terminology. In both experiments, I've very rapidly met and learned the name of people of other experiments working on similar subject. I don't know any workshop or conference where the invited scientists are not from different experiment. During these events, there are a lot of exchanges.
3) What is true, and it is maybe the reason of your misunderstanding, is that you are strongly advised to not share non-cross-checked material outside of the collaboration. The goal is to avoid biasing the independent experiments: if you notice a strange phenomena that will later turn out to be a statistical fluctuation or if you use a new methodology that will later turn out to have unnoticed systematical biases, if you mention this to the other experiment, you will "contaminate" them: they may focus their research or adopt the flawed methodology. But this is only for non-cross-checked and it does not make any sense to pretend that it has a negative impact (a lot of scientists, in collaboration or not, towards all history, don't like to share their preliminary results before they acquired a good confidence that what they saw it reliable).
4) Do you have example of things that one could not understand while it was done down the hall from them? I don't recall "not being able to understand" (the point of a publication is to explain, so people care about making it understandable). I do recall "harder to understand", but it was often from people from the same collaboration, and the reason was because of they needed to use some mathematical tools I did not know and that there was not really any other way.
I'm sure there are cases where two groups end up diverging and it makes the collaboration more challenging. But I really doubt it is not something exceptional, and that everyone in the collaborations will try to mitigate.
Your comment makes me wonder to which extend the outsiders of CERN don't have plenty of crazy myths totally disconnected from the reality. I guess it is a good example why people like Hossenfelder are a problem: they feed on these myths and cultivate them.