No, it is widely understood as a means for reproducible builds and specified at: https://reproducible-builds.org/docs/source-date-epoch/

Well, I had simply accepted that the rule for defsincdate is always triggered. I looked a bit more into it, and the cause for triggering is that Nixpkgs patches dirmngr.texi, hence defsincdate is cleared by the rule above and the fallback behaviour is triggered.
But this also means my suggested patch wouldn't help here as the modification date of dirmngr.texi would be picked up.