In normal play yes, but it always worked with golds in Arena.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 "