I've heard the phrase "more Catholic than the Pope".
Would this be an example of "more Protestant than Luther"?
Not really — it is more of a fundamental difference between Catholic and Protestant Christianity. We don’t recognize an infallible extra-Biblical human source for Christian doctrine ... in Luther, Calvin, the Pope, or the Vatican.
The analogy that comes to mind is King George and George Washington. The American Revolution was a fight to overthrow the authority of King George on American shores. But there was a movement thereafter to make George Washington a King. To replace one tyranny with another, more benevolent (at least for the time being) tyranny. To do so would’ve betrayed the principles that the Revolution was based on.
The Protestant Reformation was, at least partially, a movement which said there is no one infallible person (or church heirarchy) representing God on earth, and to object to the Catholic church’s claim on that authority. To then look to Luther and Calvin as an infallible spokesman for God would be only to replace one infallible representative with another infallible representative ... and would pretty well invalidate the movement itself.
I think its just a difference in thinking. Catholics quote Luther to us as if we look to Luther/Calvin in the way Catholics look to Popes, Catechisms, the Vatican, etc. We don’t. Luther/Calvin may have originated the movement ... but they were just as likely to make mistakes as you, or I, or the Pope. They were not doctrinally infallible.
Their theology was also colored by Catholic teaching prior to the reformation, and they were therefore more likely to accept specific Catholic doctrines (like perpetual virginity) that may not be Biblically justifiable.
SnakeDoc