As Mike said the reverse protection, safety block or whatever you wanna call it was supposed to be the delring ring at positive connection, nothing to do with the bottom cap. The hole in the bottom spring is not small enough to allow a nipple top to go through without connecting.
Problem is that it seems most everyone's ZNA positive pin sticks out enough for a flat top to work and not protect against a reverse, no matter what battery they are using. If flat tops actually didn't work, and everyone was really forced to use nipple tops, then a negative connection also wouldn't work and there wouldn't be frying boards.
I do like the option of using flat tops someday but I think it I will stick with the AW nipple tops because they do reduce the risk of confusing the direction. I am super careful anyway, I have heard of at least 3 people fry the ZNA with a reverse, two were in a rush, one was in a dark car so that's on them obviously, but two of them were using AW nipple tops, one was using sony flats.