It's assumed that "causing your next Holy Power ability to consume no Holy Power" makes the ability cost zero Holy Power, similar to the Mage's Clearcasting. It completely removes the mana cost from your offensive spells, and if you look at the tooltip during a clearcast it has no mention of any mana cost, instead of just putting "0 Mana" as the cost. It also casts regardless of if you have enough mana to cast it on a normal basis.
If it's supposed to be like this, it should say so.
Clearcasting state reduces the mana cost of your next damage spell by 100%.
Note that it specifically, explicitly, mentions mana cost.
True, it does say that, but that's just a tooltip thing. If we're to get to the bottom of this we have to go into the inner mechanics of the abilities.
Hand of Light:
Note that the mechanic is exactly the same for both spells. (Except that Clearcasting has a -1000 and HoL has a -100. Probably just the way it works. Not important.) They both reduce the resource requirement by 100%, or are supposed to. Hand of Light should work like Clearcasting in that it makes it a free, no-questions-asked, Holy Power ability just like Clearcasting makes mage spells free with no little quirks like needing the mana it would otherwise use.
This leads me to believe that it's almost certainly a bug.
Note: I won't pretend to know what those WowHead Spell Details mean. I just barely understand them to the point that I know how spells work.