Why does list[::-1] not equal list[:len(list):-1]?

The documentation you're referencing is the tutorial, which gives only an informal overview of Python syntax and semantics. It doesn't explain all the details. You'll note that the tutorial page you linked to doesn't even discuss negative indices.

The actual documentation is given in the library reference under sequence types. Although it is a bit terse and not easy to understand on a first read, it does clarify that for a slice [i:j:k]:

If i or j are omitted or None, they become “end” values (which end depends on the sign of k).

l[::-1] is the same thing as l.__getitem__(slice(None, None, -1)). Since the start and the stop are both None, the list will be traversed from one end to the other. The step argument determines the direction as well as the step.