Update on my own question: it's somewhere in between...it's a long corridor that connects the far side of the lounge to the buffet/bar area, with signs at each end saying something like "This area reserved for Centurion Card holders."

Speaking of poor behavior...the number of people who looked at the signs and then just walked through as a shortcut to the bar anyways was a bit mindblowing. Asked the manager when she came to "check on me" (which was a nice touch I've only ever seen at SEA) and she said they've given up saying anything because so many people feel it doesn't apply to them.