whether or not you consider this attack specifically, and attacks like this generally, to be terrorism is very strongly connected to how one defines terrorism.
until there is any sort of acceptance or consensus respecting what exactly 'terrorism' encompasses, these disagreements will continue.
I remember taking a history course in my undergraduate degree, focused entirely on terrorism. It was called "the history of terrorism" or something like that. and we spent a good chunk of time trying to define what, exactly, terrorism is. We were unsuccessful.
for me, I don't think this attack, absent some sort of political motivation or ideology, constitutes terrorism. but I don't begrudge those who feel otherwise. I just think terrorism is mostly limited to acts of violence designed to cause fear among the public and further a particular ideology (not just violence for the sake of it). and even if this incel thing is true, I don't consider it to be any sort of ideology or have any sort of central organization beyond a bunch of losers in their basements on reddit and 4chan.
my two cents at least.