Theoretically true, arcniz, but like most large commercial jets Concorde's rudders are neither aerodynamically balanced nor significantly mass-balanced and are driven by non-reversible (at any survivable speed) hydraulic PFCUs, so the chance of a change of mass/shape of this order causing destructive flutter is effectively zero.
That's not to belittle the significance the issue, which is taken as seriously by the airlines and manufacturers as the regulatory bodies, but just to underline that the lack of panic on their part is due to the minimal hazard rather than a cavalier attitude.