The function was rewritten in GPDB, and its behaviour was changed to also
return 'true' if the expression contains an Aggref of the given level.
That change in behaviour was made back in 2006, as part of a commit
containing a lot of subquery optimization changes. I could not find an
explanation for that particular change, and all the regression tests pass
without so I assume that it has become obsolete at some point over they
years.

This smoothens the way for future merges with upstream, by reducing the
diff in both code and behaviour. Also, you get a more accurate error
message in a few cases, as seen by the changes to expected output.