That's a completely different interaction. Sneak Attack deals extra damage when you hit with an attack, and the effect of green flame blade includes making an attack. Agonizing Blast adds your spellcasting ability modifier to the damage you deal with a cantrip that deals damage. The question is not if True Strike counts as an attack (it absolutely does), it's if True Strike deals damage. And I think the text is pretty clear on the matter - it does not. It allows you to make an attack, and it modifies the properties of that attack (including what damage it deals). But the source of the damage is still clearly the attack, not the cantrip.