"The young man wants to descend from the altar."
Translation:De ara iuvenis descendere vult.
But there are phrases like ex equo delabi (to fall off a horse, Livy), in which the person is surely no more 'inside' the horse than the putative young man is 'inside' the Duolingo altar.
There are some phrases in the OLD that are said to illustrate the meaning "from the surface of, from off" and "down from": including, in Ovid's Metamorphoses (1.261) "ex omni nimbos demittere caelo" (to send down clouds from the whole sky); note the combination of prep. ex even with a verb compounded with de- . And Cicero has a sentence about someone hurling himself ex the highest wall ( se ex altissimo praecipitasse muro). Looking under ab, there do seem to be instances of ab + places with the meaning of "away from."