Look the guys family up and see if any old letters/ documents are around and compare hand writing. But I agree the paper would be pulp after a few weeks.
Now a possibility is that the bottle was filled with sand, thus removing any air, to make the bottle sink. I didn't see anything in the article that stated that, but then again I didn't really read the article. 8>) I merely went in, to find the note picture and post it.
So, I will wait and see what becomes of this story. 8>)