So let's make a quick resume of all arguments against him and their counters
-"Ridley was shown at the boss section" He was shown in the yellow devil section and his comment was vague as it could mean a) Hazardly could appear in the pyrosphere or b) Ridley, who is a boss character like bowser and ganondorf, may appear in other stages, as playable
-"Sakurai may have not shown ridley because he is incomplete" Sakurai didn't have a problem in showing the phantom slash and the super leaf when they were incomplete, this argument is a bit funny because a lot of things he has shown is still in development, aka, incomplete
This is the way I see the situation: Sakurai made his homework and knows that ridley is one of the biggest controversials characters in gaming story and decied to reveal him in an unexpected way, he first teased people with a multi meaning commentary, then showed his shadow before he showed the FD version of the pyrosphere so that would make people not notice the lack of lava in that version I'm almost sure that Ridley will be revealed when he is about to show the boss hazards, something like this
"I'm going to explain another boss now (shows ridley in battlefield) but first let me show you our new metroid newcomer
Ridley would be unexpected and would make hype, and that's a winning combination