From Merriam-Webster. Opioid: a natural, semisynthetic, or synthetic substance that typically binds to the same cell receptors as
opium and produces similar narcotic effects (such as sedation, pain relief, slowed breathing, and euphoria
I would say it fits that description pretty well. There are multiple chemicals in kratom that have effects in the user but the most abundant compounds and the ones that give the effects people are after definitely are Mu receptor agonists. Opioids display most of their effect through mu receptor agonism.