It is close to Fargate. But on Fargate there's still a lot you have to manually control, like when to spawn up new instances, etc.
Cloud Run has this extremely simply concurrency model: You tell how many requests your app can handle concurrently and how many instances are allowed to run any given time.
* Do you mean that you could write a regular Flask/PHP app and it will automatically make it serverless? What about long running tasks that gets triggered by http? *
It's Docker based. So it doesn't really care. It just spawns up your docker file and expects your app to listen on $PORT.