With some googling I found what you're probably seeing in your grammar book:
We also use could to say that somebody had the general ability or permission to do something:
* My grandfather could speak five languages.
* We were completely free....
The example must have confused you a little, since one would say speaking five languages is a real gift, not "general" in the sense of "generic", as one of the dictionary descriptions goes.
However, in this case "general ability" means more or less "the skill" to do something, not that it wasn't something to be impressed about. It does not take a stance on whether that ability is impressive or not, just something he was able to do.
Hope this helps!