Actually when we considered our policy at QOTO this is one point we considered. IT turned out to be untrue.
Most importantly, no, no one can boost a post from GAB and have that post viewable on an instance that blocks gab. If gab is blocked then boosts of gab will also be blocked even if the instance boosting it is not blocked. So this simply isnt true.
More importantly none of our users actively boost or support users from GAB. In the years I've been running QOTO I have never seen a single boost or interaction in a positive way with a single GAB user.
You have to understand our stance goes beyond just the fact that a user has the freedom to see the reality of what goes on in the internet rather than burying our head int he sand and pretending it doesnt exist. We beleive evil/racism/fascism in darkness flourishes and it dies in the light. Defederating such instances forces those instances into an echochamber state where they are no longer attacked for holding disgusting views but instead all praise eachother and stroke their egos. By remaining federated and letting their message be public (and thus challenged by QOTO users) is the only way to combat such messages and prevent the causes from growing.
We learned our lessons from history and we wont repeat them, we will need let fascism and hate thrive again by ignoring it and walking away.