Steering shaft length protruding out of steering column tube is indeed longer on post-83 R4s and "phase 2" R5s, the spline diameter, taper and number of teeth being always the same. Maybe this hub is longer at its rear end and would foul the dashboard or steering column covers on earlier models, so that's why they refer to is as incompatible for these models.
I don't have experience with R12 steering wheels, but some years ago I got a scrap R8 one to make an adapter to fit an early 3-bolt steering wheel on my '75 R4 and used the splied hub of the R8 steering wheel - it is the same spline as on the R4.