We have multiple sources of information, each apparently official, contradicting each other. From AA's EXP Membership Guide, p. 22:

"Your Emerald symbol on your AAdvantage Executive Platinum card is your key to added comfort and luxury when you travel internationally [defined in footnote] with any of our oneworld airlines. On the day of your oneworld flight, you will be welcome at more than 340 exclusive airline lounges in the oneworld network..."

No mention anywhere, including parts I didn't bother retyping, of access being limited to airports from which your international flights, or any other flights, depart.

Since different versions of the rules say different things, I'd suggest finding a set of rules you like and carrying a copy. For me, that's the set quoted above.

I think this point has been sufficiently belabored. I don't plan to post on it any more. I also plan to continue to use Flagship Lounges on arrival as I have consistently in the past. If anyone else doesn't want to join me there - it's his or her loss. (I must confess to never having tried to use a BA lounge, or any other lounge, at JFK while planning to fly out of EWR. No lounge, with the possible exception of a transplanted version of CX Wings from Hong Kong, is nice enough to get me to go to JFK if I don't have to.)