BUG Galio's W reduces True Damage (from Olaf E at least)

So I just played Olaf top vs a Galio. I noticed that my Es were not doing their full true damage to him. Shocked, I realized his W was mitigating them. I was not previously aware true damage could be mitigated except by towers with no minions around them. This might be intended as the text of his E reads, "reduce all incoming damage by x percent. Now, a similar damage mitigation ability,

Alistar's R, also provides damage mitigation. However, on the lol wiki page (http://leagueoflegends.wikia.com/wiki/Alistar) page it says: "Unbreakable Will... The damage reduction occurs after armor and magic resistance. TRUE DAMAGE WILL NOT BE REDUCED BY ANY FORM OF OF DAMAGE REDUCTION"

Galio's E text on lol wiki does not show this. (http://leagueoflegends.wikia.com/wiki/Galio)

I don't have the means to test whether Olaf E is also reduced on an Alistar with his ultimate active, but if it is not, then this is an inconsistency and a bug.