Absent some reference to a statute under which he could have been charged, but wasn't charged, then the word "terrorism" is just sort of thrown around without any definition.
To charge someone with a crime, they have to commit the acts which make up the crime.
Here are the US laws relating to "terrorism":
https://www.law.cornell.edu/uscode/text/18/part-I/chapter-113B
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§ 2331. Definitions
§ 2332. Criminal penalties
§ 2332a. Use of weapons of mass destruction
§ 2332b. Acts of terrorism transcending national boundaries
§ 2332d. Financial transactions
§ 2332e. Requests for military assistance to enforce prohibition in certain emergencies
§ 2332f. Bombings of places of public use, government facilities, public transportation systems and infrastructure facilities
§ 2332g. Missile systems designed to destroy aircraft
§ 2332h. Radiological dispersal devices
§ 2332i. Acts of nuclear terrorism
§ 2333. Civil remedies
§ 2334. Jurisdiction and venue
§ 2335. Limitation of actions
§ 2336. Other limitations
§ 2337. Suits against Government officials
§ 2338. Exclusive Federal jurisdiction
§ 2339. Harboring or concealing terrorists
§ 2339A. Providing material support to terrorists
§ 2339B. Providing material support or resources to designated foreign terrorist organizations
§ 2339C. Prohibitions against the financing of terrorism
§ 2339D. Receiving military-type training from a foreign terrorist organization
Of the various substantive crimes defined as constituting some form of "terrorism", it's clear that none of them apply to his conduct.
That's why they went with the charges they did.
I mean, sure, in some colloquial sense the guy is a "terrorist", but that doesn't mean that his conduct fits any legal definition of terrorism in 18 US Code.