Ìf you do it, it can still be OOP, its just not in an OO language. People have trouble separating using a paradigm and using a language focused on the paradigm, for some reason.
The entire point of OOP in every OOP language that I have ever used has been to have the language try to constrain what you can do by pushing restrictions on syntactic sugar involving objects, inheritance and encapsulation, so I would say yes. The marketing claims that people will be more productive at programming by using these.
Yes, you need to have that to have an OOP language. OOP is object-oriented _Programming_, it's about how you program, not what features the language has.
In hindsight, I had your remark confused with another remark insisting that struct inode_operations is a vtable, despite it having what would be static member functions in C++, which are never in vtables, and there being no inheritance hierarchy. If you are disciplined enough to do what you said, then I could see that as being OOP, but the context here is of something that is not OOP and only happens to overlap with it. The article mentions file_operations, but ignores that it has what would be a static member function in C++ in the form of ->check_flags(), which is never in a vtable.
I'm also thinking that these kind of vtables in the linux kernel are what would be implemented by the compiler in C++. But because its self-written, you can be much more creative and do other things, that weren't possible if this would be created by a compiler.
Of course you could implement the same in C++ and then it can't be the same as the vtable introduced by the compiler, so you would just end up with to vtables, you own and the one introduced by the compiler.
If the kernel were written in C++, it would still be done the way it is done now. C++ does not allow unimplemented member functions and the ADTs currently used do. You can emulate that with multiple inheritance, but it is an inferior way of doing this.
As I said, these are NOT vtables. The fact that you and some others keep thinking of them as vtables misleads you into thinking that this can be done using the object oriented tools of C++. It cannot without major hacks and the result would be slower, harder to read and only something that a bureaucrat could like.
If the kernel were written in C++, it simply had the incentive to be less creative. Since it isn't it can be. It's just a restriction imposed by C++, not a restriction in the loosely defined paradigm of OOP.
> As I said, these are NOT vtables
Ok, you just define vtables differently then me. To me a vtable is a table of virtual functions that are used to implement polymorphic behaviour of objects. This applies to their usage in the kernel and the article. Feel free to introduce a new term for this. If your only distinction is whether these are created by a compiler, this is just a distinction I don't care about.