Hmm, not so sure now you put it like that. I'm sure I read that either here or on the CT forum before things there disappeared but must have been at least 4 or 5 years ago. It kind of makes sense to me that putting water into the bottom and letting it come out at the top will ensure it's full rather than just trickling through. And you don't really want to keep the hotter water in, you want the heat to be extracted from it, which is easier if the matrix is full. But then that's the opposite of how the radiator works. The workshop manuals that I've seen do all show the feed from the bottom though. Whether that's for consistency or efficiency, I don't know.
Don't suppose there are any thermodynamics experts around?