That is an honest take from Fed. I look at their careers like this:
Fed's early dominance made him slow to adapt his game to Nadal.
Nadal was the consummate grinder who was immediately embraced.
Djokovic used being overlooked and underappreciated as his motivation to constantly get better. The other two weren't really chasing anyone. Novak was always trailing behind them so he always had somebody to target or a number (Slams). He was able to keep that relentless motivation and drive as a result.
In a way, being the youngest child helped him the most.