You seem to believe that his hatred for gay people and his admiration for ISIS coincide? The articles written about him show him as a hateful man with well documented anger issues and known feelings about Islamic extremism. I don't see how crediting his motivations to ISIS specifically serves a purpose, when he wasn't a member of ISIS, they didn't plan the attack, and when they don't exactly hold the patent for Islamic terrorism. But hell, if it entertains you to just keep typing "ISIS", then have at it, otherwise I don't get what you're doing here.
He, himself, pledged allegiance to them. He was obviously influenced by their brand of extremism. You don't have to be physically present with someone/something to gain significant influence from it. What he did was exactly what they wanted. I assume he active on their forums, watched their videos, studied their ideology, etc. That said, you are correct that it doesn't matter what brand of extremism it is (ISIS, AQ, whatever.) What matters is a guy was influenced by religious extremism to the point where he perpetrated an act of terror that is the worst mass shooting in the history of the US.