Slop as a letter is a thing already
https://www.axidraw.com/
Also, I imagine it's not impossible to reliably distinguish between an autopen and genuine handwriting. The company who's site you linked say their machine can't perform complex pen movements so calligraphy is impossible.
The real advantage of posting a letter is that you have to pay for postage, and the stamps on the envelope will indicate which country the letter is really coming from.
As I've already said in my direct reply, there's still the various costs involved in posting the letter, and if the community is based on a particular geographical area e.g. "Connacht Hill Farmer's Association", then it would raise eyebrows if the envelope had a Kazakhstani stamp on it.