In 3E that's actually how it works. In 3E, Solar Charm's aren't 'things' themselves, but are more codified examples of a Solar's supernatural skill, mainly for ease of game balance than an a 'a solar can do this and no other'.
Hmm, one option is that you could write up a charm that works as a stepping stone towards Flawless Impenetrable Disguise with more drawbacks and not quite as many strengths and have your Solar use that to show they are really good at it but not quite at the level where they can pull of the things you can do with FID.
Hmm, one option is that you could write up a charm that works as a stepping stone towards Flawless Impenetrable Disguise with more drawbacks and not quite as many strengths and have your Solar use that to show they are really good at it but not quite at the level where they can pull of the things you can do with FID.