He is correct on that. Making "stub" cuts, which are called heading cuts, is a really bad thing to do on trees. Yes, it is common in the DIY world, but it's not a good thing for tree. Shrubs, it's okay mostly. So if he had a branch that had some leafy tips that needed to be removed, and there wasn't another suitable interior side branch on that limb to cut back to, then he has to go all the back until he finds a suitable side branch. Sometimes that's the trunk. Without some really detailed before and after, I'd just have to trust that he did what was necessary and according to good practice. Based on how thick and bushy the maple was before though, it would not surprise me if there wasn't any suitable interior side branches to cut to. I see this all the time.
I hate it when these things happen because there might be some hard feelings. I see lots of trees, including Japanese maples, that have been left to grow without having been pruned properly on any regular schedule. "Properly" meaning something that is more in depth than what the average client is aware of and really amounts to training the tree to grow to the form you ultimately want. Sometimes in these case, you just don't have lot of options and have to make some extreme cuts. Sometimes the arborist doesn't explain that very well to client, and sometimes client just doesn't hear what the arborist is telling them. Not saying that that is the case here at all, just giving you some background from an arborists point of view.