When people say "power", they usually mean "average power" whether they know it or not.
Watt = energy/sec or joules/sec, or J/s
1 J/s = 1 W
10 J/s = 10 W
20 J/.5s + 0J.5s = 10W
10 J/1ms = 10,000 W.
When Miller says 1100 0W they mean 11000Jx30/30s or 330000J/30s = 11000W. That is, 330000J averaged over 30 seconds.
When Miller says 9500W continuous they mean 9500J/s which I take to mean a 100% duty cycle or for as long as you want.
In order to get average power you need to know the number of joules over a certain period of time. If the period of time is tiny, like a millisecond, 10000W has little, if any, practical value.
Now from the Generac spec sheet all is found is:
AC Rated Output Running Watts 7500
AC Maximum Output Starting Watts 9375
What does "Running Watts" mean? Is the machine sprinting for 2 seconds, or is running a marathon?
What does "Maximum Output Starting" mean? How long is "starting"? 1 second? 2 seconds?
In short, the Generac spec doesn't say anything specific. It is vague. Hence, no comparison is possible.