As other posters pointed out, international flights require infants to have tickets. The price of an infant in arms ticket is 10% of the fare for that class. If you want to travel in F/J with an infant in arms it is completely possible but you have to buy an F/J infant in arms ticket which can get pricey. I have traveled in int'l J with infant in arms but his 10% ticket was nearly $1000.

If this was an op-up, I'm less sure what happened, maybe as someone pointed out they won't op-up the infant in arms because they don't open up an additional seat and the infant would be traveling in a class not paid for. IMO that is a bit ridiculous, if they were going to op-up the parents anyway they shouldn't mind the infant, a complementary UG would be a different story. But DL will do it the way they prefer.

Still, it's dumb that they didn't know you had an infant in arms so they could just bypass you for the op-up in the first place.