Cladding on a stadium is a totally different safety scenario than say on a multi-storey block of flats. Thats not to say that it wont be changed at some time in the near future of course, but no building is ever claimed to be 100% safe anyway. Think of all the kitchen areas within a modern stadium and the hazards they have within them, the electric cables, lighting, sound systems etc which are all over the place including the roof and also the machinery used to maintain the stadium housed within it etc.
In the event of a major stadium fire in any area various evacuation scenarios will have been prepared for, including exiting the stadium and/or entry onto the pitch for fans. If the club, licensing authority, fire service and police are doing what they are supposed to do we should all be as safe as we can be. Sadly of course when they dont it can be horrific ie Hillsborough or Bradford. They were both dated stadiums not modern like ours is today. Im surprised Hillsborough hasnt been demolished tbh, that bloody stand is still there just reconfigured. Horrible.