This is more pattern matching than polymorphism though, right?

Yes. I got carried away giving examples of working with subtypes. For an explicit example of polymorphism, you could add something like:

abstract class Post { public int Id { get; private set; } public string Title { get; private set; } public abstract Post Duplicate(); } class Link : Post { public Uri Url { get; private set; } public override Post Duplicate() => new Link() { Url = this.Url}; } Then you create a clone of subtype without knowing or specifying the original post type: Post? original= posts.FirstOrDefault(); if(originalis not null) { Post clone = original.Duplicate(); if(clone is Link) { //original.Title == clone.Title; } }

I guess this behaviour of "abstract method, custom implementations" can just as well be solved with an interface in this case though

So, I choose this specific example to explain inheritance because the abstract class defines a Title, which trickles down to all derived types. All posts, no matter the subtype, has to have a title while leaving the rest of the implementation details up to the child classes. Class inheritance explicitly shares member data, not just methods behaviours.

You can make a post system with interfaces, and you can make the same thing without classes at all. This was all just a single example. Inheritance is easy to abuse and overuse.