My point wasn't clear. The amendments to UK law would normally breach art 27 of the VLCT but in the case of the Withdrawal agreement it could be argued that the VLCT does not apply to treaties involving the EU.
The UK has already left the EU. The Withdrawal agreement contains a number of legal traps which give the ECJ authority after December. I think the amendments are an attempt to prevent UK judges continuing to follow the development of the Factortame case, and to prevent the UK being pursued for years in the ECJ.
The Factortame case and its development in UK law are the reverse example of how long it will take the UK to extricate itself from EU law. It took years to establish the limits of EU law in the UK and it will take years to remove.