The excerpt from the resolution serves to illustrate the consensus on EU law on this matter. Those who drafted the resolution subscribe to a strict interpretation of EU law that precludes formal negotiations on future trade relationship until the UK is a third country, and MEPs, officials, civil servants and lawyers agree with this interpretation. If the UK disagrees, it will have to litigate the matter, esp. now that it has practically no credibility and goodwill. The text you cite refers to a withdrawal agreement and it has been established that the agreement will not progress until the Irish border matter is settled in a manner consistent with the December agreement. If that's a problem for the UK, it will have to get support for a new resolution amending the directives given to the negotiators. It is truly remarkable how reluctant the UK is to accept that its statements and actions have logical, predictable consequences.