I'd argue it's just a common sense prediction based on the actual in-park examples we do have to measure it against. Like
@Jerroddragon and Pacific Fun pointed out, Springfield is a verifiable nightmare on busy days. The narrowness of Wizarding World can be utterly headache-inducing to traverse when it's just filled with a sea of people. The Starway can get congested, as does the landing at the bottom where people getting on and off are pushing through the same crowd made up of people who just got off the Mummy, are stopped looking at the wait time board, are taking pictures of the JW arch, are crossing to get to the raptor encounter, etc.. The entry and bridge to the Studio Tour, which is a good comparison to the warp pipe in terms of size and purpose, can even get annoying. And on and on.
Obviously people get through eventually, but that's not the point--the point is the crowding exists and happens, and on a peak/busy day, that little tunnel is going to be slammed with people going through it. How do we know this? Again, because we have so many other examples of it happening elsewhere in the park.
Whether it's happening or not at Japan is irrelevant when their park is completely different than ours--not only did their SNW open in the midst of the pandemic, which is going to affect the crowd numbers you see in videos, but they have more park spread for crowds to be dispersed and kept busy and more walkway space in general; you won't tend to see tons of people crammed down a narrow pathway having to come to a complete standstill because a family with a stroller decided to stop in the middle of the walkway to discuss whether they should get a beer or a donut like you do heading in and out of Springfield. USH is a tight, packed, condensed park, which means crowding permeates
everywhere on a busy day, and to think that the entrance to SNW will be miraculously breezy and empty on a peak day at the same time the whole park is full of the examples that have been brought up is a little naive, IMO.