In this era of self-promotion some people actually can't recognize those things.
But I think its very obvious that he is a good programmer. His work is literally legendary.
It's unfortunate that people latched on and the "Notch is a bad coder" meme took off. I imagine the same is true with Tarn.
"His work", as in, the game he made. Game development is a multi-faceted beast. It is quite possible that while game design is awesome, technical implementation is a mess even if it reaches the goals.
I have heard DF being praised for having so many things to do, for being able to create wild stories, etc, but never for technical excellence.
DF is definitely an admirable and awe-inspiring project, regardless of the beauty of its innards
Genius is one percent inspiration and ninety-nine percent perspiration - Thomas Edison.
In fact it's pretty well known that Dwarf Fortress has a horrible code base and it would be a huge project to bring it up to a good standard.