Sagan is the personification of excellent positioning skills.
No idea about FTP, but his max sprint power is likely hundreds of watts lower than guys he regularly beats to the line.
It's all due to his ability to find the right wheels to follow (and change when he needs to), partially due to necessity since Bora Hansgrohe has no leadout to speak of.
I think a lot of people consider/focus on FTP because of the reality that the further away from the red you are during the race, the more likely you'll have a chance to do anything at all on the final lap.
I regularly hang with guys way faster than me with way higher thresholds. But I do it because I know how to draft and conserve both energy and momentum. Once they go above their threshold, there's little I can do. In a "fresh vs fresh" sprint, it's very evenly matched. In a "been riding hard for 30+ miles" sprint, I'm off the back before the sprint begins. The only way to change that is to increase my 3.4w/kg FTP.