APKMirror is a great resource but there's no way they 'manually' review each APK that hits the site.
so to detect this manually, they'd have to be checking every single app! more likely, they've an automated system that detects new APKs signed with prominent keys.
Although it's very possible that the thing you mentioned is a fluke and somebody decided to ask the question "We should probably keep all the Facebook apps definitely-safe, so which ones are signed with this key?"