The problem is, is that’s on a JP data center. What you have seen is on your data center. Unless you’ve played on NA and done BA here, you are unable to speak for what it’s like on an NA data center—I do not mean this in a rude way; you simply do not have the experience to speak on it.
Do randoms know to stay behind the Perception player? You cannot say. I have seen players run ahead of them even in Discord runs, and the leaders have to remind them “Don’t do that”.
Even in full Discord premades, I have seen incidents of players making careless mistakes (i.e., pulling mobs when they weren’t a tank, aggroing roaming mobs, triggering traps). The possibility of players that enter who are not in the Discord make those possibilities higher, hence why the Discord always attempts to pull them into VC so that they can listen to callouts for AV/Ozma and avoid making careless mistakes. If the run is an Ozma prog/clear, it becomes even more important that they get into VC, because it does just take 1 person to invalidate hours worth of work, because getting ~56 people together and into the instance in and of itself can take 1~2 hours.
Most of the time, the players that go into a BA run where there was a full premade do not actively contribute to the content. They pop defense logograms for AV/Ozma (e.g., Platebearer), they hit the bosses only enough to get credit for the kill, and then they spend the rest of the time keeping themselves alive with Cure logograms, because it is hard to get them to join parties that have free spaces, so their survival is completely up to them. Some actively refuse to join parties despite the premade trying to get them into one. However, while they are using Cure L on themselves and not attacking, they are requiring the rest of the group to pick up what they are not contributing to meet the DPS check/enrage. I take great issue with players like this, as I think everyone should contribute to the content equally. This is a group effort.
I think the people in the Discords just want things to run as smoothly as possible. It’s quite possible that, like me, they don’t have the faith in the playerbases over here to risk doing a full-PUG run of this content. But I cannot speak definitively for them. I can only speak for myself, and make guesses on what I think they feel.