I don't have the statistics in front of me, but most of the time, longer-cage RDs *do* have the geometry necessary to wrap larger sprockets than their "road" counterparts -- that's what they're expected to do on MTBs. You are correct that a longer cage alone wouldn't change the largest sprocket that could be used, but a long-cage RD that couldn't handle larger than a 26T sprocket would be kind of dumb, so I used a little bit of short-hand in my post.