I could be wrong, but I think he can compare whatever he so chooses. Could be wrong though. Maybe some rules somewhere.
What's all the fuss about? Everyone's free to make ridiculous 'apples and oranges' comparisons if they want to - no one's forcing them to be reasonable. They just won't get any meaningful answers, that's all.
Beethoven's op. 13, as a sonata, is the largest, most 'substantial' piece on the list - the kind that takes even longer to prepare than any one etude. Yet it doesn't have the kind of extreme technical demands you find in Chopin's op. 10 etudes, like the 10/4 or 10/11. Apples and oranges, that's all.
-Rach3 (I prefer apples and sonatas)