They have to follow EU trade rules DURING the transition period -- which they can end anytime. They can freely trade with the EU in goods and services with no tariffs at all
How is this different from being a member? It isn't - because it's a transition period until 2020 - for the UK to wrap up its dealings and leave.
did you even really read the withdrawal agreement as opposed to what others said the agreement was about?
You’re conveniently forgetting about the so-called “backstop”.