Technically true, but there's other factors:

1. Even if you're crouched, you may still be the tallest structure (e.g. you're in a big, open field).
2. As the NLSC states, crouching prolongs the amount of time you're in danger with little benefit over running to safety (I suppose if there is no safety, it's better than nothing, and the CDC guidance does list this basically as a "last resort").
3. Even if there's a taller structure near you (crouched or not), lightning causes electric currents along the top of the ground that can be deadly more than 100 feet away from the strike point.

Height has basically nothing to do with it - it's the accumulation of static charge in a conductor which produces the dielectric breakdown of air. Even though the human body is a relatively poor conductor, it is still a much better conductor than the ground itself. It makes no difference if your head is 6 feet above the ground or three feet above the ground - you will accumulate charge the same way.

Also most strikes on people are not direct strikes, but people being close enough to to the primary plasma channel to provide another path to ground. The electric potential required to produce an arc from 5 feet away is orders of magnitude lower than it is from 3 miles up.