Why is it incorrect?

According to the statistic, 73 million did not vote in the primaries this year and about 88 million eligible adults do not vote at all. Add to that 103 million that are not eligible for voting. So if I'm not mistaken, only 18.5% voted in the primaries.

You mention they have the highest disapproval rates, but how is disapproval expressed if not by voting?

What if they were denied the chance to vote due to onerous voting restrictions (closed primaries)? Clinton has horrible negatives, but the state party of NY forces voters to be registered by October of the previous year (!!!) for a late-April primary.

We may be using the word "representing" differently.

I took it to mean "those 9% are statistically representative of the majority of americans views/choices/opinions/desires/whatever".

Where as it seems you meant it as "those 9% represent a plurality of the votes cast in the primaries." Yes?

The people who run in the primaries are selected by far fewer than 9%. Parties other than the two have been excluded by bipartisan agreement between the two.

Maybe in the Trump case, since he had fewer than half of the people that bothered to vote, but Clinton won the majority of those who bothered to vote in the primary, so even if you think the choice of her as unrepresentative of those who could have voted in the primary, first past the post isn't really the culprit, political apathy is.

That said, I generally agree that first past the post is bad and should be gotten rid of.