Ultimatum's 'buffs expire faster' affect silver flasks

We know these:
1.flasks are not buffs,cannot be affect by expire mods.
2.silver flask grants onslaught and it can be granted by mageblood. effect like create smoke when use cannot be granted by mageblood.

So why 'buffs expire faster'make onslaught from silver flask expire faster? I think it is a mistake.