It sounds like they're talking about deployments where a new process is spawned to handle each request, but for the most part no one deploys Python web apps that way, so I'm not sure.

PHP may have some advantages in terms of deployment, so I guess it's a question of whether those advantages outweigh the potential advantages of using Python for development, which is of course subjective/situational.

It's because PHP uses a CGI process-per-request style model, while there's no good or modern for support in Python for that execution model these days (the community is built around WSGI and ASGI instead). Opcode caching isn't actually relevant because PHP reuses the processes and in-memory structures these days as much as possible.