You can make a layer and say in a comment somewhere that it implements layers a, b, c of spec X. The code doesn't have to be totally alienated from the spec, there's always a middle ground.

Some complex things may be best described very verbosely so that people can understand them. When that understandment is there, it can be trimmed down to the essential. So I don't think it's always true about the code translation, but in practice probably more often than not.

Imagine that almost no spec is good (technically: what they do is good, but they're poorly made), then you'll understand Linus' point of view to not translate specs into bad code. If you're lucky enough to work with a good spec you can get away with it, but the code shouldn't be judged by how well it resembles the spec, but on its own.