Ultimately this is a flaw in the wording of Unbound Elemental and the functionality of Guidance. My guess is, since Overload is an option you can opt into, Unbound doesn't check to see if it was an Overload card. Unbound probably triggers on play when a card is played that Overloads you, it won't check as a card is resolving its effect and happens to Overload you.
Unbound should have its text changed to match Tunnel Trogg - that way the effect is more consistent across cards that are similar. Also, I think making it a 2 mana 2/4 would have been a better change than a 3 mana 3/4. But that's just my opinion.