People who truly understand their subject should have no
trouble writing for a popular audience.
This is false for at least two reasons.1) People who truly understand their subject know they can't write for a popular audience without handwaving, partial truths and actually getting the complexity of the subject across. They may not want to write in that way.
2) The ability to 'think like a layman', properly estimate their background and slowly build up a story that they can follow without getting lost is a skill entirely different from 'knowing your subject'. It is prepostorous to suppose every intelligent person has that skill. It's a typical case where a lot of people mistake intelligence for skill and it's insulting to (technical) writers that have actually put effort into learning how to write.
All in all, I doubt whether 10% of all people that 'truly understand their subject' can write for a popular audience.