The fact that IQ and vocabulary are correlated across a population doesn't imply that, and it's clearly not true unless you don't think someone can learn a new language later in life. Or you think that a person can learn 10k new words in a new language, but not an extra few thousand in their existing language?
>Competent writers have a varied vocabulary which does not come across as forced
Yes competent writing requires some minimum vocabulary. It doesn't require a vocabulary sufficient to score near perfect on the SAT. Most writers limit the vocabulary they use to match their target audience, so having a much larger vocabulary than your target audience isn't going to be that useful.
Yet, from Wikipedia (https://en.wikipedia.org/wiki/Intelligence_quotient):
>Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect.
I don't think vocab can be taught. The reason is, vocab is very highly correlated with IQ, which is stable thought life and cannot be boosted. A large vocab requires a strong crystallized memory, which is a part of IQ. Most people when they encounter new words either are unable to infer the meaning and or forget it, but smart people do both.