You need a very high altitude for that margin. (Coffin corner), and since the aircraft did not go "mach" which would have broken it up in the air, we have to look at a possible stall. But stalling all the way to SL leaves you nowhere near the coffin corner.
A "very high altitude" is relative. All you need is to be at the maximum altitude for any given weight which is less than the aircraft's maximum certified ceiling especially in the case of heavy airliners. In my little business jet we can get to 43,000 most days but if the temperature is higher than standard and we departed at max takeoff weight it's possible that 40,000' is our maximum altitude. You can definitely see the narrow window between overspeed and stall condition with the low speed awareness really creeping up. Our maximum certified altitude is 51,000 and I've never been higher than 47,000 since there aren't really any practical situations to ever go to 51,000 in this particular airplane.
Also a momentary overspeed condition will not make an airplane break apart.
