Random Kriss Swordsman Upgrade

I was fighting barbs with my Kriss Swordsmen, when one of them (the one with the Enemy Blade promotion) got upgraded to Longswordsman. I don't even have the Steel tech yet so I'm wondering if this is something Firaxis put in as a consolation for getting the Enemy Blade prom or if this is just a bug?

You must've hit a camouflaged ruin under the barbarian encampment you had forgotten about and gotten the "finds better weapons" ruin underneath. Since you're playing multiplayer, you don't get the text box saying this.

There was probably an ancient ruin where the barbarian was in and the ruins upgraded your unit.