They weren't an organized army, open, just so happening to use unconventional means. That comment is directed to those who think Fabius Maximus led a guerrilla force because he was smart enough to not be baited into ambushes or battles he didn't choose. Same for Sertorius, or countless other generals, who used novel and controversial tactics, completely outside the box, but who still were fighting conventionally.And yet… that is from where the term was born and applied
Small groups of Phase 1 and 2 insurgents in the Peninsula War were actual guerrillas. That was the ONLY WAY, at the time, they were capable of fighting.