Thanks for your comment.
You’re right, those mixin classes are not the same pattern as inheriting from multiple interfaces, which are also called mixin.
Composition is a simpler solution, so to be preferred when possible, as you suggests. Inheritance can be useful for instance if the base class has protected methods we want access to, or virtual function we want to override.
And I learned the term (and the pattern) in the talk from Arthur referenced in the article.

You will need some kind of reference to be able to update the original class. I would use composition instead of inheritance in this case. Wondering if defining just a function that takes a reference to the original class doen’t provides a good solution in most of these cases.

I’ve used the term mixin as something inherently related to a class. A class is composed in some way of several mixins, not the opposite way, something that decorates a class. Do you have some references where the term mixin is used as you use in this post?