Upvote:2
No, the Tibetan deities are not real in that ontological sense. There are two ways I will answer your question. One is to think of these deities as the personified qualities. To thus humanize them serves to make them a model and aspire to become them, aka, those qualities. Second, thinking along your lines of Yarnell's definition: Ascertaining emptiness is synonymous to saying that one is able to see the mereological relationship in terms of dependent origination. And what does dependent origination conclude to a question about appearances and reality? What appears is not how it is. Therefore, in ascertaining emptiness in the form of deity is to take that as the object of meditation whose emptiness is analytically established and thus not revere something doubly illusory but to know it as illusory and yet use that to arrive at analytical conclusions. Hope this helps.
As to the implicit question of offerings, those rituals have their uses. What one may think about an activity might well be hocus pocus but that might not be a characteristic of that activity to begin with!