That's where the controversy is. They never actually outright say he was a boss, they say he was a 'boss character'
Technically, lots of the playable cast are boss characters (dedede, ganondorf, bowser)
Also, the section he was shown off in was the 'Stages' section. Not hazards, not bosses.
Lastly, there was a lot of small coy stuff going on that video, between Sakurai's trolls on ZSS and Palutena, and the fact that they both cut the volume on the section where they show ridley, and never actually show him, or mention him by name
So while definitely not confirming he's playable or not, it certainly throws some sand in our eyes.