Edit: Those who are low in skill benefit more from college than those who are high in skill because there is a saturation point (if this were false we would all attend college multiple times). If "elite" firms are looking for the "elite" people who were already at the saturation point before coming to college, then for the population they want it is probably true that attribute (skill+intelligence) causes signal (degree).

Employers who do not need to hire the most "elite" employees need not behave this way.

As an aside - i don't think this is college specific. I think all signals - indeed, all causal lines - follow the format you just laid out. Suppose you wanted to Signal an Attribute. People will treat your signal as evidence for the attribute if they believe one of the following things:

Attribute influences Signal (preexisting skills enable college education, health enables large peacock feathers)

Signal influence Attribute - (college education increases skill, muscles cause strength)

Hidden factor influences Signal and Attribute. (intelligence enables both college education and skill acquisition, hormones increase both secondary sex characteristics and fertility)

I can't think of any other Signal-Attribute configuration.