Here are some easy to implement design suggestions: You should change the title of the first heading so it's not the same as the banner. I think "What is Tarsnap?" is a good choice. Speaking of the banner, vectorize it. Finally, please remove the bars from the asides.
Your product is good, so transferring a little attention to its web presence is worth the effort.
I hope he doesn't listen to the folks saying he should follow the herd. Websites where you have to page down below the fold to get basic information on the product? We should be pointing them at Tarsnap. But flat design is king on mobile, so we won't be getting many of these website layouts anymore.
On small width device the menu will be on the bottom, so there's no duplication. (website seems to be responsive)
I like that the menu is on the bottom and not hidden behind a hamburger button.