You are perfectly correct
I was absolutely not at 100%. it's not easy to determine the proper power you can sustain for 20 minutes.
I know for sure that my FTP is somewhere between 250W and 300W. I have already done a climb that was almost 60 minutes at 251W. Then I had some more strength to keep riding for another 60 minutes (and I did not puke!). But all my numbers show that I'm still <300W. That's Strava power.
Then the next question is:
is Strava power accurate?Well, after watching a GCN video where they ask the same question about Powermeters I get my answer.
The short answer is "Yes, Strava power is pretty accurate on pure climbs provided that you have properly logged your weight and bike weight". Don't rely on it for flat or descent.
If you know a climb (length, elevation) that you have timed yourself on, you can calculate pretty accurately your power, here (look in the "hill climbing menu):
http://www.wolfgang-menn.de/GCN used this tool to validate their powermeter!