While your power computations might be correct and the manufacturer's insinuations might be in question, I am extremely doubtful that any sound track ever evenly demands 100 watts sent to all speakers equally. (unless of course you are already pushing well over a hundred, average, with your mains.)
I expect the workout for the surrounds is extremely light compared to the mains. The sub has it's own amp so is not a concern. I'm thinking that most times it's only the mains and sometimes the center that get pushed hard for anything more than a quick blip.
I guess one exception is if you are using a stereo or mono to all speakers mode on your amp. Most people don't have a need for this, but it's possible.
Our amp experts should verify or debunk my guesswork. I'm OK either way as I'll learn from it.
