I agree, it's a sensible guideline, but he would be critical of websites that don't follow this guideline.
It's good to be consistent. But lots of sites now have an colour scheme that makes it clear which links are visited and which ones are unvisted. Generally the visited ones are a duller / subdued.
I used to read a lot of Jakob Nielsen's writing, on his website and books. I don't read his work so much now because I find what he says tends to be too conservative. He tends to be too strict with guidelines. For years he insisted that all unvisited hyperlinks should be blue and all visited ones should be purple.
One of the tricks with usability I've found is knowing which rules to stick to, which ones to bend and which ones to break.
It's good to be consistent. But lots of sites now have an colour scheme that makes it clear which links are visited and which ones are unvisted. Generally the visited ones are a duller / subdued.