The only reason that I can see that the gear cable guides are not going under the bottom bracket is that it's a Cinelli cast BB shell; I've never seen routing going underneath these, but on the other hand, I've not seen that many of them.
I think most Cinelli cast BB shells would have the cables going under. In the 70s when over-the-bb-routing was the standard, Cinelli cast BB shells were not all that common. By the time they were used on a lot of frames, routing under the bb had taken over. Look at all the Cinelli "spoiler" BB shells, I bet not one of them had over the BB routing.
At that time, before lo-pro frames and aero bars, a TT frame would have been not much different to a road road, perhaps except for closer clearances and steep angles