It may be true locally, with a given power grid that is not expected to get an upgrade. It may also be the unquestioning acceptance of a rule-of-the-thumb number. (And even that number is for an average.)

However, a study for one of the two Danish grid operators found that 50% wind is possible with only minor tinkering. And that's wind alone: if, in theory, you combine similar large capacities of wind and solar, there is actually a 'natural balancing' between the two that reduces the combined intermittance.

_{*Lunatic*, n.
One whose delusions are out of fashion.}