Yep. Just like Orc traits specifically state “when I take damage to life”, but all traits that center around taking generic (ie., not skull) damage (Frenzy, Aspect of Humility, Orbweavers third trait) all only trigger when taking damage to life. Conversely, traits that trigger when taking skull damage trigger when the opponent matches skulls, regardless of what was hit. Traits that trigger on skull damage have a bunch of different terms used that doesn’t necessarily track with how they function, with functions including “only works if the attack resulted in damage (including to armor)”, “can trigger every time skulls are matched, regardless if attack 0 or any damage was dealt”, “triggers if skulls are matched and my attack is greater than zero, damage dealt doesn’t matter”, and “triggers on every skull match even if I am not in first slot”.
In this case, they just didn’t need to specify Mana Steal separately from the others, since without this distinction, the reasonable assumption would be that Mana Drain immunity would make you immune to mana steal (including from the OP). Humility/orbweaver/frenzy traits just need the words “to life” added to them. Skull trait terminology and functionality is a bit of a rats nest, though, and they really need description passes for everything, including the usage of keywords like “transform” referring to two different mechanics, ambiguous lethal damage terms, and a way to filter where you are searching for your keyword (so you can search for things like “elf” or “orc” or “war” or “poison” and get the troops you are actually looking for).