I'm not sure this engineer wasn't the "most brilliant" contributing to FFmpeg, because there are a lot of smart folks that contribute to it, but he certainly made significant contributions.
His reason for leaving is that big tech companies overwhelm people like him with security theater. In his own words:
> most of the secrecy around security issues is just theater. All the "best practices" like OpenSSF Scorecards are just an attempt by big tech companies to guilt OSS maintainers to make them work for free