Evolution is climbing mount improbable. And natural selection (even in the face of in this case intelligently designed selectors) funnels organisms into potential dead ends which they can’t evolve out of - they’ve gone up the ‘north face’ and in order to reach fitness in the face of their selectors, they actually need to be on a completely different mountain.
An antibody that dead ends all current evolutionary trends for an organism means that they can’t evolve out of the dead end natural selection has funnelled it down.
So no, this isn’t a ‘might make it worse’, this is a ‘end of covid’ agent.
That’s if the antigenic trigger can be made into a vaccine. More likely what the research is describing is that it is now possible to make a monoclonal antibody that can work as an infusion and is active against all known and future possible variants with the same efficacy.