From what I have been reading, for Boxscars you would want to use TCA CROSS. Subcision is for rolling scars. I'm not trying to contradict your doctor. Just repeating information I have heard. Maybe subcision will work on both types of scars, but the gentle slope of a rolling scar seems like it would respond better to subcision. With the harder edges of a boxscar, well, it's harder to imagine that untethering that scar will raise it to the surface and it would then lay flat. But I don't know for sure.
I went to a few consultations and most of the docs told me subcision and filler. I have read the same information as you and was confused by their reco but hey, I'm not a doctor so who was I to question their advice, right? Well, maybe I should have. The filler has lifted the scars but hasn't filled them in, if that makes any sense. Improving the scar above my lip was worth the money to me but overall, I am disappointed. However, I refuse to give up! I am writing to my doctor this week to see if I might be a candidate for excision or TCA cross.