The point of the article being that you should show a message saying "Username/email is not found" if you can't find the username/email in the database.

I agree on all the "Pros" you listed.
The problem I had with those IDEs is that they are really not as powerful as offline IDEs.
If you are using a powerful cross-language code completion with code intelligence every other minute, refactor, use different code navigation, search and replace in files options etc ... then you're gonna a have a baad time ..;

Isn't it going to create massive flight delays if used at large scale ?

Since you checked in your first flight they are going to wait for you on the second one, making multiple "last calls" hence delaying every other passengers in the plane isn't it ?

This is not going to make flying any more enjoyable ...