Regardless of whatever definition you use, this was a deliberate act
Terrorist attacks are usually carried out in such a way as to maximize the severity and length of the psychological impact. Each act of terrorism is a “performance” devised to have an impact on many large audiences. Terrorists also attack national symbols, to show power and to attempt to shake the foundation of the country or society they are opposed to
The size is not the issue nor the determination of the moniker. These guys selected a soldir, a political tool and spewed religious rants along with the "get out of my country stuff"
= terrorism