It could be worse than that depending on what happened legally when an existing UK air service agreement such as with the US was superseded by a collective agreement between the EU and the third country.
It could be worse than that in that many countries didn't exist before we joined the EU, so there is no agreement to revert back to, though clearly we are getting away from the EZY discussion now.
Getting back on point, I'd be skeptical of any airline saying they are Brexit Proof - more like, "we have now done as much as we practically can do based on the limited information available"