I reported 2 elms this year out in leaf and also 15 maples, just didn't get to everything in time, didn't loose a single tree. I also collected 9 Buxus in the middle of summer on a 45 degree day and only lost 2. It's more about after care and root disturbance I think. Any collected/potted tree gets sprayed with seasol twice a day. If the roots have been reduced then they will struggle to take up water until they recover. Misting the foliage and trunks helps to reduce moisture lost, even spraying the trunk of no leaves helps IMHO. This works for me and I loose very few trees. I choose to dig at optimum times for the health of the tree but not always possible.
Everyone is different and bonsai don't have rules but rather guidelines. For newbies following these guidelines to begin with will help establish confidence to then push the boundaries later on.
I think recommending deciduous trees be repotted in winter is good advice to a newbie, but seeing that red maple hasn't done that its about giving advice to give the tree the best chance.
I remember repotting a tree then asking then question to be told it was the wrong time to repot. Someone gave advise on saving the tree and I managed to save it. I learnt a lot from repotting that tree, when it SHOULD be repotted and that fact it can be repotted outside that time with care. Made me push the boundaries on other trees to.
Sometimes we need to take ourselves out of the question to give the best advise for the person asking the question.
Just my
Josh.