Isnt this in some sort of banal recognition that twins are more complex? Originally they were and it was sound doctrine at the time, but its not such a hard and fast rule these days. You can "jump" from a Cirrus to a DA40 as long as you have a "glass endorsement" without type training (even if you would be reasonably stupid and probably would not get insurance) but you cant "jump" from a Aztec to a DA42. In reality in terms of differences I doubt there is much between the two. If you have flown enough types mind you I dont think there is anything intrinsically complex in terms of handling or technical differences that would mean with a comprenhensive self brief you couldnt make the transition without difference training but even then an hour with someone current wouldnt be such a bad thing. On the other hand even with tail wheel experience I would wager most would struggle to "hop" into an Extra without some time on type.