As far as the 'very few pure-born Garleans are able to use magic' could either possibly be an error (given that there are acknowledged minor typos and errors in the Lorebook), or it could be merely a narrative embellishment, 'very few' being shorthand for 'none at all'. After all, it has been repeatedly stated even on this forum by Ferne that Garleans are simply incapable of manipulating aether (read: using magic) in their bodies and that it would in fact prove fatal for them to do so if they tried.

Of course it's also just as possible that it's exactly as it states and that one in a million native born Garleans might find they've been blessed (or cursed, as more likely the case for a Garlean), with the ability to manipulate aether and thus use magic, just as there is the possibility of Garleans becoming awoken to the Echo, but the statistical likelihood of that would probably be extremely low.

Either way, their lack of magical aptitude is the Freudian excuse for instead developing magitek technology and half the reason for their zealous crusade against the Primals and the beastmen - it's simply Garlean insecurities and a need to "compensate for lacking something else."