Of course it is, as you yourself put forward. How else could it be considered a "grey area", something you "wouldn't normally do", except under the pressure of the situation (namely that I was about to match you in VPs and thus "close to winning the game"). If barbarians attack your province, it is not open to me, conventionally under a NAP, to take it while your attention is elsewhere (and then be defended from any counteraction by the supposed NAP), nor is there any "one turn rule" for dibsies. If this were not so, it would be possible to siege someone's cap w/ whom you supposedly have a non-aggression pact, or block paths of retreat in a war with a third entity (or interdict pursuing forces, as could have happened in the Eriu situation, as I almost sent forces into two of those territories to finish him off). No NAP could hold under such circumstances. Somewhat similarly, as you implied, no NAP can really hold under a situation of imminent victory, either (
"force majeure").
But, as I said, I basically agree with your right to do so, though you could well have cleared up this question in advance before taking action, as that would have allowed me to operate under equivalent conditions
