Inspiration support gem question

So, i 100% get that inspiration cannot reduce the cost of reserving spirit, it is clearly laid out in the description. However, the description is also very clear that the gem reduces "cost" This is so vague they had to add the spirit disclaimer. My question is, should skill like unbound avatar, which have a cost other than mana, but not explicitly spirit, be affected by inspiration? Inspiration is explicitly recommended for unbound avatar by the game, and can be slotted without a problem. If it is not intended to work with this skill, it shouldn't be able to be socketed as it cannot be with pure auras that have no mana cost like herald of thunder.

So am I way off/misinterpreting? Should we just assume they mean mana cost explicitly?

Unbound avatar doesn't work with inspiration I think its safe to assume its just mana/hp cost reduction

Appreciate the feedback, thank you! I think it may be helpful for GGG to add a tool tip for this as not only is inspiration a recommended gem for unbound avatar, it can be slotted in without one of those "this gem doesn't work with this skill" errors. Whats strange is I'm having the same problem with tempest bell.