With CFM engines, this means that with a dual engine failure in a cruise at high altitude, the optimum relight speed may be quite a way above MMO, and a progressive increase in speed towards 300kts during the descent may be required to avoid exceeding MMO.
Would like to add that, at that high of an altitude, you'll be above the max guaranteed relight altitude anyway. Once at or below that altitude, flying at the optimum relight speed won't be a problem.