I would still classify that as object oriented behavior.

You have a class/type of objects, and you have a set of functions or operations which are associated with (loosely or tightly) and operate on that type of object.

For example, I would say that file descriptors are a handle to a type of object. They encapsulate the thing that they represent (file, DRM buffer, eventfd, whatever), and some of the functions that operate on them are polymorphic. For example, read, close, ioctl, etc., don't care if the file descriptor is a file, a queue, or whatever.

You use syscalls to interact with file descriptors, but they are just as much object handles.