What he means is that there are rainbow tables available for many MD5 hashes. There is software that can search hundreds of thousands of possible hashes per second. You don't need to calculate the MD5 hash over, you just have to do a simple text compare, followed by a lookup in the rainbow table.
If you have a rainbow table of the major hack sites in which you're interested, I bet it doesn't take more than a second or two to determine if the hash you sent is of one of those sites.
Maybe that doesn't fit your definition of easily enumerable, but it fits mine.