I think the point here is that having a class.isClass() function that didn't recognize normal functions as classes would be very similar to having a Function.isFunction() that only recognized arrow functions as functions, and returned false for anything else. This would just be confusing and inaccurate.
The "old way" for classes is just as obsolete as the "old way" for functions. Neither is deprecated, and both are still widely in use today. There's use cases for all of these. So, we must recognize both kinds of functions as functions, and we must recognize both kinds of classes as classes.