Earlier quoted context omitted.
You do have to consider that g is the amount of plasticity though, which is mainly genetic. A better way is to think of it is that genetics provides a potential capacity which may or may not be fulfilled. Training helps individuals to varying degrees.
That seems intuitive to me, but lots of other things in science seemed intuitive because I wanted to believe them. If the measured IQ difference in individuals can be overwhelmed by simple factors like “have I taken the test before”, we don’t really have a useful empirical measurement to say these things and we’re just stating our hopes and dreams.
Measuring g is hard and taking shortcuts is tempting. A reasonable repeatable g factor test takes hours, and is too often replaced by a single test. There are ways around the test-retest issues but they are roads less travelled.