A mage PC in the play-by-post Dungeon Fantasy campaign I'm playing in cast Body of Air on herself, in order to rise above a brewing combat. She then tried to cast Stone Missile, but the GM commented that she couldn't do that because Body of Air gives the "No Fine Manipulators" trait while in effect (hands are made of, essentially, air currents). He said she wouldn't have trouble throwing the missile, because that's an "innate attack" (additionally, the missile of a missile spell isn't material according to multiple game designer clarifications I've read), but that she couldn't cast the spell.
By standard rules, with skill 15 or higher, a PC caster should need to speak one or two soft words, or make a quick gesture in order to cast a spell (except for spells that specify some special movement or incantation requirement), and it's been my understanding that the PC in question has base skill 15 or higher (not my PC, so I'm not certain, but the OOC discussion reinforces this belief).
Is there a rule I'm missing that would prevent a PC under the effect of Body of Air from casting a spell that needs only a couple words?