It is not a CAAB requirement. They have no power over permits. It is a Labour requirement.
You will not find it written anywhere. Such is the way things work in Botswana.
As far as I know... Many pilots had work permits rejected for 'insufficient hours'. After much trial, error, and heart break, a meeting was held between Labour and the operators. It was at that meeting that the operators were told "500 hours for expats".
At least one operator is actively fighting this.
While it's true that in LFB's original post he mentioned that personality is more important than hours, that was written many years ago. Things have changed. To some extent it is true- personality will make you or break you... So long as you have the minimum hours.
The other thing to be aware of, as I've mentioned before, is that many operators have a minimum hour requirement on their insurance policy. I know that Moremi Air and Major Blue Air need 500 hours TT. Safari Air is 400 from memory. Even if Labour reduced their requirement, insurance minimums are not negotiable.
All this being said, a few weeks ago Delta Air offered a job to a 350hr pilot. Permit applications take a long time, so it will be a while before we know if this guy is successful or not.