Not everything is within its competency though, some elements are within the executives competency - though Parliament decides who forms the executive.
Hence Parliament did not invoke Article 50, it gave the power to the PM to invoke Article 50.
Parliament can not revoke Article 50, it needs to give the power to the PM to revoke Article 50 then the PM must at their discretion exercise that power. If the PM refuses to do so, Parliament can No Confidence the PM and either choose a new PM or trigger an election after 14 days.