I'm no lawyer but I expect that selective bit of legalise refers specifically to the part of the withdrawal agreement that covers trade, and specific types of trade. The Tories have been clear and no one has challenged it, that if the backstop is invoked we do not have to pay EU fees, we have control of our borders and we can make our own laws all while having tariff free access to the common market.
The fishing bit is nonsense probably based on a perverse interpretation of the agreement. From that right wing rage the Guardian:
Fisheries
The EU has repeatedly said that it would only allow British seafood exporters tariff- and quota-free access to EU markets in exchange for a reciprocal agreement that EU fishing fleets can continue to operate in British waters.
This potentially explosive issue appears to have been kicked down the road: the withdrawal agreement essentially says only that the EU would apply tariffs on fish until a separate agreement was struck on access to EU fishing in UK waters.