Reasonably sure that's not true for this quest, else we wouldnt need Dragons Nests for big rituals and magic crafting.Exalted path sorcerers can't use mana, as they have essence instead.
Anyway, doesnt matter whether or not Molly uses mana for Dresdenverse sorcery as long as other supernaturals do.
Mana Path allows the adept to sense and manipulate mana in themself, the environment and in other supernatirals.
If you want to deny power to enemy sorcerers and supernatural beings who draw on that power, this looks like a useful trick.