Yes, the video uses C++.
Obviously if one compiles a library then the compiler has no way of knowing that other subclasses of `shape_base` do not exist. My point is that when compiling a binary as they are doing for their video, the compiler knows that there are no other subclasses that it needs to cater to.
It might require LTO explicitly, of course. At the very least godbolt doesn't devirtualize without LTO [1], but godbolt itself breaks if I enable LTO [2] and I CBA to test locally right now.
The fundamental problem here is that C++ doesn't have a good abstraction to represent visibility of public types, since any other translation unit - even across the DLL boundary! - can re-declare the type and then derive from it. The only way to constrain visibility is to use anonymous namespaces, and that only works if the type can be confined to a single unit (that C++ compilers seem to ignore the optimization opportunities here in practice perhaps indicates just how rare this actually is).
Refactorings are moves you can make. Choosing when to make them is up to you. In fact, Fowler provides guidance along with each refactoring suggesting when it might be applicable (i.e., not always)
What compiler are you using that devirtualizes every class hierarchy? I suspect that Casey is using C++ so he may (unfortunately) have multiple translation units in his program.