It was written by hand, in a rather poetic, inspiring, and exciting handwriting! At least, I find it so.

The bottle likely helped preserve the paper and the writing. In addition, I'd guess that they used good quality paper, because Iron Gall ink - which was in vogue at the time the letter was written - is fairly corrosive and can eat through paper.

Addendum: I also find the bland (under)statement of their work comforting and inspiring.

Isn't that just how people were taught to write at the time? I had always heard that children were taught to write differently in the uk than in the us, but all those letters were exactly how I was taught 90 years later in the us. The r varies a couple places but the one used more often is how I was taught. I was taught with a ballpoint/biro, but with a flexible nib or brush pen it comes out looking like that, just not as pretty. I'm not sure if that is iron gall. There is a little bit of water damage in one spot and I've never seen real iron gall move around at all onve dry. You can use watercolors or even alcohol markers over it.