See the slide for full details, but the important thing to note is that the entry points in the interface class are implemented inside the fifo device and are bound, at a higher level, to the calls made by the writer device. This kind of plumbing of upcalls to entrypoints formed an essential basis for future transactional modelling styles.
However we soon run in to the well-known OO problem with multiple instances of an interface: not often needed for S/W but common enough in H/W designs.