"effecting some change in the subjects that makes them more capable of sending the costly signal of graduating from college, which is an absolute improvement"

It depends. Consider a government subsidy for college tuition. This increases the number of people who go to and then graduate college, but it also makes the signal less costly.

But I basically agree with "it's more complex than it seems to determine what's actually positional". The difficulty of determining how much of an observed benefit is absolute vs positional is a lot of what I'm talking about here.