I would guess this was an idea proposed by the real estate developer which would increase the value of land and lot in Riverdale. Some of the earlier plans for the Henry Hudson Memorial Bridge did have the bridge East of its present location. That is East of the Inwood Ridge keeping it lower but crossing over to where the Major Deegan highway is today. This was proposed to keep Inwood natural forest and the Riverdale Ridge free of a major highway.
But the builder Robert Moses did care that the HH Pkwy would cut right through the Inwood ridge and divide Riverdale much like the other Moses who parted the Red sea Robert Moses forced the Bridge and HH Pkwy and parted the communities. More detail information on the building of the HH Bridge can be found in Robert Caro’s excellent book the Power Broker.