It is to do with the legislation hierarchy, where rules in EASA legislation applicable to both EASA and non-EASA aircraft have been removed from national legislation to avoid duplication (and possible errors/differences). As you've found Rule 5 can now be found in SERA.5005, plus a few exemptions in
ORS4 (No.1174 returning us to the old Rule 5!). You will find that the AIP has been similarly butchered.
ifitaint...