Sounds like you misunderstood. They didn't say they are merging PRs after a few seconds. Just that the difference between a good one and a bad is often obvious after a few seconds. Edit: typos
I'm not sure there are many ways to interpret "I know whether I want to merge a PR within a few seconds".
Exactly, every PR starts with:
1. What’s the goal of this PR and how does it further our project’s goals?
2. Is this vaguely the correct implementation?
Evaluating those two takes a few seconds. Beyond that, yes it takes a while to review and merge even a few line diff.