There's no way that's true, the ccp had at least thousands of journalists, authors and published professors in prisons and detention camps 50 years ago.
There was even more extrajudicial killing at that time, though, so maybe they were only detained a few days before being executed?
Fun thing is if Pen's criterias were a little looser, Israel would be way ahead at the top of the list. Not even accounting for capita. Palestinians routinely get arrested for Facebook posts.
For example between 2015 and 2018, at least 470 Palestinians were jailed for "incitement".