"Strong" is relative. A lot of people are strong in different ways, I think.
(Anecdotal sample size of one
For me, I average about 220 watts average on my commute. 45 minutes each way each day. That's 90 minutes per day at 220 watts. People say I'm strong. Get me on a hill though, and I get dropped easily. I'll churn out 4-5 minutes of 350+ watts and the skinny-mini's just walk away. They are strong...except they are not. They are light, they are fast, they are aero with the deep rims & they can sprint. Add 50 pounds to their bike to equalize the difference or ask them to keep a sustained 20 mph average minus all their aero and they just sort of cramp up & peter out.
I guess it's turtle v/s hare. Sprint v/s endurance. I like to say: "Anybody can buy their way into fast, but you can't buy strong."
I guess a stronger rider is anyone who is faster than you or inspires you to apply effort. In our local club rides, we all agree we are strong riders (22+ mph) but where everyone is strong is different.