Borders in the sea are still borders.
We can debate/suggest/argue until we're blue in the face, the fact is that changing the EU membership of the UK does affect the GFA. It was known before the vote, and it was known afterwards.
What it wasn't, was considered sensibly by anyone in the Leave campaign or who voted to Leave.
It was brushed off as threats from the IRA whenever anyone dared suggest that the GFA (and therefore the ceasefire) was being compromised by the stupidity of the government.
Because it's all very well us sitting in our cosy homes bleating about war. The fact is that it isn't us who'll have to live with it again, and our lives won't be devastated by it.
But there are many who will be, and up until now they've been an afterthought.
For the thousandth time, it's up to the government to be honest and say there can be no brexit without ripping up the GFA therefore there can be no brexit.