Plutarch fabricated this form of punishment as a way to paint the Persians as viscous monsters.

They’d have to be pretty vicious to do this, which included pouring honey — a viscous substance — on their victim.

Snark out of the way, people used to be a lot worse to each other than they are now. Read Stephen Pinker’s Better Angels of our Nature; the first half is a catalog of all the terrible things people did to each other (painful just to read about), the second half is a set of interesting theories as to why we’re less terrible to each other now. It’s clear that most people are not aware of how much better things are now.

Then there’s the possibility that Plutarch, in the tradition of Herodotus, was simply repeating myth and rumor that had been related to him. Herodotus was pretty clear that he was getting his information verbally, and second hand, if he couldn’t have first. Plutarch was less clear, but, really, who knows?

BTW/FWIW, this conflict had a group of Greek mercenaries on the losing side. What became of them is detailed in Xenophon’s Anabasis, which is IMO the best adventure tale to come out of that period. Aaaand I’m pretty sure Alexander, son of Philip, used the Anabasis as, basically, a how-to-conquer-the-Persians manual, which, as we know, he did. Fascinating period. So glad I’m not living through them.