I don’t think that he stood up for the EU in any special way. If he would say that US support Croatia’s bid to NATO, it would sound strange. Reading between the lines, all people would understand that he oppose their entry to the EU. WHILE I’m almost sure that the Croatian president in his opening speech mentioned the EU.
No, it could easily be interpreted as meaning that he takes no position on it, because the U.S. is not a member of the EU and so therefore we're neutral on it. That's a perfectly respectable position for any country to take.