A bit late, but I think Ridley's speed isn't necessarily much of an indicator of his status, considering how bosses now play the role of stage hazards. He's moving rather slowly in comparison to his appearances in his series, sure, but considering bosses are going to show up in the middle of regular matches and interfere, Ridley being so slow could easily be a design choice to prevent him from being ridiculously intrusive rather than just... largely intrusive? (Stage bosses seem like they could be too distracting in general; kinda hoping there's a way to turn them off)
Point is, the design considerations for a traditional boss that fights against a single player (or perhaps multiple co-op players) until one side is defeated (with no additional or further goals afterward) are, by nature, going to be rather different from a boss that's meant to show up in the middle of a regular versus match and act as what is essentially a sturdy, violent pinata whose reward will ultimately go to only one player (as far as we know, anyway; we only know the details of how the Yellow Devil works, it might vary a bit with other bosses). With the Yellow Devil, for example, beating him isn't the goal of the match when he shows up; it's still a contest of points and/or survival between the players, and time running out or taking off your opponent's last stock will end the match regardless of whether YD's still around. Note YD's patterns and movement; he takes awhile to move from one side of the arena to the other (giving even the slowest characters plenty of time to get into position), has predictable attack patterns, and doesn't move while attacking, allowing the players to wail on him or their opponents without worrying about him suddenly going after them or anything. A lot of this behavior can be attributed to mimicking his original boss fight (though toned down and likely much less challenging due to the radical difference in gameplay between MM and SSB), but this behavior is regardless well-suited for his purpose in Smash; intrusive, but not so overwhelming as to prevent gameplay from continuing mostly normally.
With that context, I don't think Ridley's slow speed or him stopping and doing nothing for several seconds are very strong points against him being a stage boss; were he to fight very similarly to his main series appearances, constantly moving around quickly and attacking mercilessly, it would present several problems. He'd be incredibly overwhelming whenever he appeared, demanding all players' full attention lest they get blind-sided by him while trying to take out their opponents; slower characters would be at a heavy disadvantage to get the last hit while faster characters would have a heavy advantage (King DeDeDe would struggle to catch up and deliver a single blow while Sonic might be constantly smacking Ridley in the face the entire time, as an example). These might be fine for a one on one fight against Boss Ridley, but when Ridley's defeat is merely a bonus rather than the goal and the players are already concerned with taking each other down, I can't see it turning out all that well.
Thus, I can see Boss Ridley being that slow, as well as him stopping and doing seemingly nothing for several seconds. He wouldn't be the main attraction; that's always going to be the fight between the players. Like the Yellow Devil, he's there to impact the fight, not completely consume it for a minute or two.
Mind, I still think Ridley yet has a good chance to be playable after all. There's enough odd things about the clip for reasonable doubt, I feel, and him being playable is simply a more fun thought to entertain than the alternatives; not a whole lot to get excited over with them, after all.
One more thing: The odd animations are a good point in playable Ridley's favor, but it is worth noting that the only other boss we've seen in action so far is the Yellow Devil, who is deliberately limited in animation to match his appearance in Mega Man. While Brawl's boss animations were smooth, we haven't seen any other SSB4 bosses in action yet, so we don't quite have the frame of reference necessary to say this is unusual for this game's bosses. Unusual for past Smash games, yes, but that may turn out to be true of all of the bosses.