I'm certainly open to the very real possibility that it isn't an agonizer scar and that Lorca isn't the MU version. I'm just speculating at this point since that's pretty much part of our entertainment while we wait for the next episode.
As to whether an agonizer leaves any scar or if it does what type of scar it leaves -- at this point I don't know. Yes, it looks kinda like a taser effect, but since this is sci-fi I really do not know at this point what sort of energy it uses on the victim and what effect said energy has on them aside from the obvious intense pain caused.
I just thought that it was an interesting theory, since the TOS agonizers are indeed roughly the same shape and size as Lorca's back scar. And are you sure that the scar is a keloid one? At least to my untrained eye it looks like it might be a hypertrophic scar since it is still rather thin and has not expanded.