To be honest who's to say it's not their side but your side, or even in between both of you. And I say that because the API call might have died half way before sending to them.
I agree that employee shouldn't be working for them if he is swearing, I've never even done that myself but I can be pissed off when told it's our fault when it's the clients.
But they even try, and to threaten to tell a competitor is pathetic, they could report it on FraudRecord with a valid reason if they have one lol.
Good luck never had any issues with either companies myself.