It depends on what those aspects are and what game they're in. And if the PC can create aspects, then that does not contradict with "only using the abilities that PC has at their disposal" because creating aspects would be an ability they have at their disposal.
The other thing, as has been said, is that the game assumes that you are working within the fiction and not trying to play on godmode. So if the player can create aspects--again, whatever they are in this game--then they are doing so within the boundaries of their PC's abilities, background, etc.--which to me still counts as only using the abilities the PC has at their disposal. And as I said, the ability to decide or create things wholesale, like what the runes actually mean, is not necessarily a common ability in these games.