He's totally right. I remember it being the weirdest film I'd ever seen. I saw Bitter Moon in the theatre, with some friends of mine from the Parking Lot. We often got off work with just enough time to make it to one of the local art houses to catch the late show. I'm pretty sure I saw this in high school, which means technically, I snuck into the theatre to watch it, but by this point we were regular enough theatre goers that no one thought to check us for ID. I think there were maybe twenty other people there. No one walked out. Even when I thought things couldn't get any weirder, and then they did, as the movie cut to Peter Coyote crawling around on the bedroom with floor wearing a pig snout playing a twisted barnyard sex game, the audience was as quiet as Ebert described them.
When I watched the film, there was even a moment when the reel snapped, or ran out, and the theatre announced there would be a fifteen minute delay and offered refunds by way of apology. No one left. No one asked for a refund. Bitter Moon wasn't bad, it was just beyond.
Once someone criticizes a film, it's easy to gang up on it, but Ebert's quote was great, because it didn't leave anyone off the hook. It recognized that Polanski had crafted a story, however bizarre and taboo, that nevertheless enthralled us.