The federal building in Portland wasn't empty. Although court was not in session, there were federal employees inside. Garland's ridiculous answer actually goes against the definition of 'terrorism' as defined by the FBI:
"The unlawful use of force or violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives"
https://www.secbrief.org/2014/04/definition-of-terrorism/