[ Slightly paraphrased for clarity ]

"Both optimism and pessimism are incorrect, both are too ideological and lead to not doing enough.

Extreme optimism means you don't have to do anything, extreme pessimism is defeatist: we are going to lose, nothing to do (..)

outcomes are up to human agency"  

(minute 26)

I do think that its better to be a definite optimist in Thiels words as it more likely to lead to success.