I has been my experience that code headers like this are always stale in any non-trivial software with more than one author, so I don't think it matters at all. It's actually news to me that we even have this guidance. I would simply remove the section, personally, but I would also be OK with just bringing it in line with current practice.
Tangential to this, the License section of the Community page on swift.org says:
However, there is no file named CONTRIBUTORS.txt at the root of the Swift project, nor indeed anywhere in the project at all that I can find by searching.
There is a file with that name at swift.org/CONTRIBUTORS.txt, but it is not on Github and it also claims to be auto-generated. Moreover, I know that I personally have made pull requests which were accepted to the Swift project, but my name does not appear on that list.