The problem with this assumption is that one doesn't have to be aware of how their own brains work in order for them to work.
The fact that people seem to be universally bad at assessing probabilities means only that however their brains work, that mechanism doesn't produce intelligences that are finely adapted at assessing probabilities. The machinery that is producing that intelligence could still be completely probabilistic itself.
Humans are fantastic at pattern recognition, but that doesn't mean that our intelligence must have been created by a pattern recognition algorithm.
We can't say that humans are fantastic at pattern recognition either. We may only be sure that our built-in pattern recognition is better than our ability to create pattern recognition algorithms.
The fact that people seem to be universally bad at assessing probabilities means only that however their brains work, that mechanism doesn't produce intelligences that are finely adapted at assessing probabilities. The machinery that is producing that intelligence could still be completely probabilistic itself.
Humans are fantastic at pattern recognition, but that doesn't mean that our intelligence must have been created by a pattern recognition algorithm.