That's not correct. I read that sentence as saying that you walked into an empty room and sat at the table. Nobody else was involved.
We'd say this if someone seated you: "I walked into an empty room, and then suddenly a ghost appeared and sat me at the table."
I think the confusion is with "we were seated at the table." That could mean two things - a waitress seated you, or you just "were seated there" - as in, you hadn't immediately sat down; you'd been sitting for a period of time.
Yep, you're completely right. I got that confused - tinkerdan got it right:
I might read the British author::
He was sat at the table.
And what they would, in all likelihood, mean is that he was sitting at the table already and continuing to do so; but in my twisted American English I would immediately picture someone taking him to the table and forcing him to sit or perhaps just leading him there to sit.
In British English - certainly colloquially - it's common to perceive "was sat" as nothing more than past tense. Whereas in American English "was sat" strictly implies a third party acting upon the person sitting.
More of an explanation for this here:
BBC Learning English | Ask about English