I thought I wouldn't learn much from this video, but I actually learned a lot because it connected some ideas in my head and revealed relationships that I haven't seen before, great content!
Beware that inheritence does NOT imply sub-typing, in general. You can in theory have a Dog class inheriting an Animal class such that you cannot upcast an Animal into a Dog.
Good video and explanation overall. Only constructive criticism I have is the emphasis on downcasting which is generally a bad idea, although I can see why you went that route. A better way to get access to the concrete type is to put the code that depends on it somewhere with more context: Where you have access to the this-reference. I'm assuming here that your videos here is only about languages with single-dispatch, since you mentioned this-references. For single-dispatch languages with a 'this' or 'self' reference, you would typically use what's called the Visitor pattern or at least something of the same flavor. A situation where this is useful would be in the example you mentioned with an array or collection that uses subtyping to get around arrays being homogeneous. A visitor implementation would allow you to perform some operation on the array that requires knowing the subclass of each individual element, but be more maintainable and less error-prone than downcasting.
The funny bit is that this is heavily biased towards Haskell's opinions on types, and polymorphism. Not a bad thing, but gives an argument for replacing OOP with FP, while clarifying why OOP leaves you exposed to various problems.
The video uses terminology that is particularly popular in functional languages but that terminology is still appropriate and correct for imperative languages. In fact, I can't think of a single imperative language for which the statements made in this video are incorrect.
This is like going to a biotech conference and complaining about why everyone is pro-vaccine. Type theory is a research domain led almost entirely by functional programmers and researchers. Alan Kay's OOP instead advocated for extreme late-binding and dynamic typing. C++, Java, and C# merely inherited the need for at least a barebones type system from C.
