It’s more the “easily found” part. I haven’t even read the links, not to speak of the literature on this, but i’d guess that the upper bound proof is pretty theoretical, and may not be taking into account all the things that would cut down on the possible size.
ETA: In other words, that upper bound may be about that much of an overshoot, just because of the limitations of any method of finding such an upper bound. It’s a surprisingly tough problem.
