AGG_PLAIN thinks sorts are free

AGG_PLAIN sometimes does sorts, but it thinks they are free.  Also, under explain analyze it does not explicitly report whether the sort was external or not, nor report the disk or memory usage, the way other sorts do.  I don't know if those two things are related or not.  

This behavior seems to be ancient, at least back to 8.4.

Does someone more familiar with this part of the code know if this is a simple oversight or a fundamental design issue?

Here is a test case, in which adding a "distinct" increases the run time 500% but doesn't change the estimate at all:

analyze foo;