This is indeed a terrorist attack according to the State of Nevada.
That to me is a pretty useless definition. The term terrorism is used to help define, a coordinated motive, ideological base, political objective etc. in order to better frame and bring into focus those who would potentially commit another similar act of terrorism. It sets apart an ideology from a grievance for example.
I think people are too eager to jump on the "oh white man = mentally ill vs brown man = terrorism" bandwagon without actually thinking about what terrorism is and why language, yet again, is so important.
If you go around labelling everything as terrorism then the term looses it's meaning and usefulness in the sense I've already mentioned.