I was really replying to RobertP - timelag!
There is a very fine cut-off point when a glass plate ceases to be of use as an ambrotype and is of more value as a negative. You can have heavy ambrotypes and light negs. So you surmise correctly - it's the dark backing that reverses it, but if you are a fine technician like Quinn, you will have made your ambrotype to a suitable density in the first place.
Usually, like you say, a change in technique and developing is called for if you are going for one or the other, but - we all make mistakes! If I get something usable and it looks good as an ambrotype but is too light for a negative, then I'm happy to have an ambrotype. You can tell that I'm not much of a technician.