To some extent I agree with you Moliere. However, the list is based on most Klismos' preferences and I find that many enjoy anywhere from 1 quart to 5-6 quarts or more (heaven forbid I should try that). The list was, in part, more for fun than serious consideration. But if you want to get really technical and "by the book" (text book), then for the most part 2 quarts is still considered the norm for an adult "high volume" enema. Many of us here don't always follow the norm.
To answer the question "What volume is a real enema", well, you can read that question several ways. True, a "real" enema is any liquid injected into the rectum, but I don't think that's what the original poster meant. Rather, what volume would really do the job for that individual is the way I interpret the question. For many klismos, a real "enema" (note I changed the quotes) can be any maximum volume one is comfortable with.
Myself, for example, a 2-quart soapsuds enema followed by a 3-quart enema with saline solution is the norm. Anything larger than 3 quarts will spell trouble for me. For some, that's nothing; for others, it's a challenge just to get past a quart (and I AM talking adults here). Enema volumes are typically based on height and weight, condition of the colon (healthy vs problematic), practice (starting with 2 quarts and working up to 4 quarts on a gradual basis, etc), enema solutions (what you put in it), and so on. Many variables can dictate volumes one can tolerate.
Again, the list is suggested as a fun thing but some folks just might find it useful.
By the way, I am a long-time confessed klismo and I have been on many enema enthusiasts groups, chat boards, etc for many years (even wrote stories and dabbled in enema artwork a while back) and only just joined this site.
Alex