Buen día is a legit, grammatically correct, although uncommon greeting in Spain. Buen día is also more logical than buenos días to a non-native speaker. Kovačić is not Spanish, so you could understand why he would have a problem knowing the local customs regarding greetings. Also, to me it sounds like he pronounced it more like buon than buen or bon, which is also understandable since he just came from Italy, where you say buongiorno. He said it in Spanish, it just didn't turn out perfect, calm down. Why would he even know how to greet in Catalan?