I think he's a little bit right, if you use his terms.
Art is subjective. It has to be. What constitutes art is just as subjective as a work of art itself. I think some games are more art than some movies. I'll take BioShock over any movie Meg Ryan has ever been in.
Ebert's reasoning seems flawed upon closer examination. It seems to me that he thinks games cannot give the subjective experience that a film or book can, intimating that everyone gets pretty much the same experience playing a video game and that there's no room for personal interpretation of the thematic elements within as everything is linear and set up the same way for all players. I can understand this to a certain degree. Two people playing Super Mario Brothers are more than likely going to have the same experience.
He also seems to either be ignorant or dismissive of the different aspects of making a game. Behind the scenes of most game companies probably looks a lot like a movie studio when making a film. There are scripts, set designs, costume designs, music score, actors...the only difference between cinema and many games is the level of user interaction, and that may be what he believes the barrier to be, which seems weird. Art is supposed to get a reaction and draw the viewer/reader in and challenge their preconceptions.
If a movie suddenly paused while you were watching it and gave you the option to decide what happens, is that movie ceasing to be art? If the creator of the work intended it to be one way and that was scrapped in favor of giving the viewer a choice then, yes, I guess you could say the creator's artistic vision had been compromised. However, most games these days are written with numerous endings and paths that are just as valid as as any other one. In fact, that's what's so great about games with a great narrative; you can go back and do things differently and experience the story from a new point of view. Movies are not so fluid. Stories are told in the same 3 act structure that plays were written in centuries ago, and when someone breaks with that tradition and steps outside that structure they are hailed as geniuses (Tarantino, Christopher Nolan)when all they really did is tell a story the way they wanted to.
The long and short of it is that some games and movies are crap, some are good and some are art. Just because it's art on film does not mean all film is art.