Both could separately be true—Brexit could enable a socialist agenda in the UK at the same time as the EU is reconfigured to the benefit of its member nations more broadly.
I note though there's no actual evidence put forward in that piece as to why Corbynism would be legally threatened by the stability and growth pact.
France is the nation in the best position to alter the trajectory of the EU, and with its perennially sluggish growth and mounting economic problems, has an urgent need to do so.
The benefits that Germany has obtained historically from the structure of the EU are the weight that will eventually bring Germany to accept reform proposals if the alternative is France's departure, or a different kind of breakup.