It has always only worked for bronze. Also, you normally cannot have duplicate gold cards; only in Arena.
You are right that it should be added, it may be clear that Henselt is intended to only target bronze units, however the descriptions cannot just ignore the existence of a valid game mode and the ability should work as described.In normal play yes, but it always worked with golds in Arena.
I have a feeling the 4.0 patch broke this or fixed it if it was never intended. Bronze should probably be added to the description then if that's the case.
"Mobilization (former Henselt's ability) can no longer be used when there are no valid targets on the player's side "
Holy shit that's a complicated one! xD Guess I shouldn't have picked it for Arena then. :/ It really should be explained better!You first need two or more copies of the same bronze unit in your starting deck to even have the option of activating Mobilization. Then, you'll need to draw and play one copy of the unit while another is still in your deck in order to activate your leader ability each game. I have it brick on a regular basis when playing constructed; I can't even imagine having Mobilization as your leader ability in Arena.