By publicly posting his manifesto online, he has made a clear political statement, coupled with a destructive act of violence; that certainly fits both the dictionary and common-use definitions of terrorism. Even though he hasn't explicitly called for others to support him, and doesn't appear to have any ideological agenda beyond what he stated in his letter, he still wanted people to know exactly why he did what he did.
This is not the work of a terrorist organization, which uses violence towards an ideological goal; it was an isolated incident committed by an unbalanced individual. This was also true of McVeigh & The Unabomber (among many others) but the term still fits the intention of the action itself. Nonetheless, to say "this was not an act of terrorism" obscures the difference between the two, and reinforces the inherently-racist connotation of the word itself.