I assume this is to have more control over compilation process.
Without annotation processor, you can expect your code to be compiled and behaved in an obvious way.
With annotation processor all bets are off, your code and code that results from compilation are completely different entities.
So with this switch being explicit, you might enforce politics like lack of annotation processors for better clarity.
While security theoretically might be better, in practice with modern build tools there are enough ways to cause code execution, so it probably doesn't matter much.
Without annotation processor, you can expect your code to be compiled and behaved in an obvious way.
With annotation processor all bets are off, your code and code that results from compilation are completely different entities.
So with this switch being explicit, you might enforce politics like lack of annotation processors for better clarity.
While security theoretically might be better, in practice with modern build tools there are enough ways to cause code execution, so it probably doesn't matter much.