That doesn't mean the entire sky is covered in thunderstorms up to 53,000 feet. They could be isolated, or embedded, or in a nearly solid line. At any rate radar can paint these storms very effectively, so they can be avoided (with a competent operator).
The weather is reported by air traffic control as "tops up to 53,000 feet"; "isolated area of thunderstorms with tops up to 53,000 feet", etc.
But for sure, if one has tops that high, it has severe turbulence within it.
I think he said ‘tops up to 53,000 ft’, and that would explain the turbulence they were dealing with, and of course they couldn’t go that high....so they must have tried to fly through the storm right?
Now he is correcting himself at 52,000 ft.