If you are saying Irish Republicanism, is about nationalism then I agree it is a pile of chite, because in my view nationalism is a pile of chite.
I disagree with your premise that departure from the EU could have happened with the safety of the GFA 'assured'.
Unless you mean it would have been some kind of pretend departure, what used to be called brexit in name only. The more sophisticated might even call it a 'customs union', which to my mind would not be 'leave' as voted for.
I repeat for the millionth time, if 'departure' or 'leave' means two different systems side by side then there is a border in between. That is what puts the GFA at risk.
Again there could be an unspoken 'turn a blind eye' situation, which pleases the Ealing Comedy Whisky Galore fans I suppose, but in that situation eventually it would be pointed out that the Emperor has no clothes.
However I do agree that if a united Ireland comes out of all this it would almost be by accident, but it wouldn't be brexit as voted for.