Knuth is solving different problems than Joe Average programmer.

I don't think anyone will argue against reusing existing databases, web frameworks, etc. when you are writing a web app. But if you are, say, writing a new typesetting system... who cares about reusability? You just need to make it work. The components aren't typically used elsewhere in that case, so your priority should be writing a good typesetting system, not writing reusable parts.

I think that's what he's saying, anyway...