I think what they mean is that they can't stick it in with ease.

Even though my girth is 5.25", I can't EASILY slide my dick into a TP roll. I have to squeeze it and twist back and forth kinda fast to get it in.

I think if it's much more than an effort to do this, which is believable with lots of the sizes I've seen so far in this thread, they just give up quick and assume they can't stick it in, even though it would probably be possible with lube and dedication.