I agree there can be difficult hairs to split and they're best split by the people involved. This isn't one as far as I see. There are practical onesies and babyish onesies. If you only like the practical ones, there's no hint of AB stuff. If you like the babyish ones, there's probably some fire with that smoke. Until you can find a practical purpose for frilly stuff, I think that's a pretty clear indicator (until informed otherwise) that there's something sissy or LG going on there.
I think even a pacifier can be an eye of the beholder thing, although aside from the rave scene, the people using them for non babyish purposes is pretty small. I do see it as an AB thing for myself, although I don't generally find it regressive. It's more a sensual thing and sometimes I wind up using it just because I like it and not because of any AB associations that I can observe.
I think the best way about it is to ask the person if you're confused, since it's not like these are scientific labels. I consider myself ABDL as a single, indivisible unit but we have people who clearly identify as just one or the other. I do think these labels have value but we shouldn't be hamstrung by them.