That is, more or less, true. Project-GC does support the "corrected coordinates" field, but due to the way that Groundspeak's API is constructed it is not practical to use that field in general (there is no way to figure out which caches a user has corrected coordinates for).
Thus, if you enter corrections in the "personal cache note" field, Project-GC will detect them automatically. If you enter corrections in the "corrected coordinates" field, Project-GC will only see them if you specifically as for them. The way to do this is to put the caches in a VGPS and then export them as a GPX, making sure to check the "refresh corrected coordinates" checkbox.