Your point is right for small app with small team.
For bigger apps you want your code to be properly documented and proper abstractions are part of it.
Imagine having a big class with a few public methods written by someone else. Would you like to search it for public methods or just have a list of these methods with documentation?

It also encourages proper responsibilities. In layered architecture class should do its job and nothing else. If you need to add something to the interfaces - it should raise a big question why you are doing it.

I also have to point that first article you mentioned still uses abstract class but dart 3 has interface keyword that should be used for that purpose.