what weather are you about to have. Mine have been in the ground for about 3 weeks. Mid 20s wont hurt them. Might lose a couple small ones. I usually plant 100+ and hope for 75%
There is a risk, I believe, that multiple transplant shocks may cause the plant to think it has gone thru a winter and go to seed. But it would depend on a bunch of variables (like how far along with bulbing it was, temps etc).
It may be that Dixon thinks a few extra days in the fridge is less stressful then starting to grow in a pot and then being xplanted again.
I have pot them in pots and tray many times were the weather has been bad with no problems at all. Just be careful with there roots. I have put them in 2-4 inch pots and then made a hole the same size and just popped them in when the weather is warmer.
I planted my onion sets in middle November and they are growing very well. I am in the Austin area and we have had a colder than normal winter with temps in the teens for a large part of December and January. The great things about onions though is that they produce a better crop if they are in cold weather during their early growth then have an increased temperature. The onions that I planted were the Texas 1015 Super Sweet, so they are specifically developed for this area.