I see no way that this frame is not authentic, but here are the details: the seller says that the frame is made of steel and that the "titanium" label was a factory mistake.
Note the last picture - is that Italian on lower label?
I can't find anything on the internet re Marin bikes with Columbus steel. It seems that Columbus steel is/was used mostly for road bikes. This is supposed to be a mountain bike.
I believe the higher end bikes at least did use Columbus steel: 97 I think the catalogue is on here if not I have it somewhere. I have seen a 97 (think) Columbus Pine Mountain.
My 1999 Team Marin is Columbus Cyber. I don't know all of the different grades of steel but I was told that it's a good material. I definitely like the way it rides.