Which Algorithm did OpenAI used to create a hide and seek playing Agent?
This is specified in the original paper that led to that video:

Policies are optimized using Proximal Policy Optimization (PPO) (Schulman et al., 2017) and Generalized Advantage Estimation (GAE) (Schulman et al., 2015)

They, nevertheless, used concepts often used in evolutionary algorithms, specifically competitive co-evolution, as the goal of the seeking agents is contrary to the goal of the hiding agents, and this drives the reinforcement signal.