As stated, it very well could be an old husband’s tale of entering erect. I too subscribe to this method and pump accordingly.
In my experience and despite what’s considered common here at TP; I enter as erect as possible 90-100%, slide into the pump and then I relax, focus on deep breathing and I avoid kegeling. I find when I allow my body and thus my dick to relax, I get the most expansion and length.
Although I also don’t milk the tube, I will tug up on it, similar to a wade fisher occasionally tugging at his rod to keep fatigue to a minimum against my (almost non-existent) fat pad. If I literally just hold/let it sit there and don’t move the tube at all, it ends up getting sore 30-40 minutes in.
I hope this helps!!!