IMO g is purely an abstraction. As long as the rate you learn most things is within a reasonable bound spending more or less time learning/perfecting X impacts the time you spend on Y, resulting in people being generally more or less proficient in a huge range of common cognitive skills. Thus, testing those general skills is normally a proxy for a wide range of things.

LD breaks IQ because it results in noticeably uneven skill acquisition in even foundational skills. Meanwhile increasing levels of specialization reward being abnormally good at a very narrow sets of skills making IQ less significant. The #1 rock climber in the world gets sponsors, the 100th gets a hobby.