In other words, leading the Euroleague (and any other league likewise) in scoring doesn't mean you are either the best player or the best scorer. Neither is it any kind of a guarantee that you'll succeed or fail in the NBA.
I'll leave it at that.
Of course, leading any league in scoring mean that you are one of the best of that league. You cannot be a scrub and leading at the same time your league in scoring.
Also, about X-Man and Tarpley: Tarpley actually destroys European Cup competition, leading the league both in scoring (26 ppg) AND rebounds (15 rbs) during 1993 season. Next year he average 21 ppg+13 rbs playing for Olympiakos. And this was 3 years after his last NBA game.
He come back in the NBA for 94-95 season, scoring 12 ppg.
Xavier McDaniel? He was 34 years old, coming from an 8 ppg season with Boston. Then he scores still 18.4 ppg+8 rebounds in Euroleague, #5 in boards x game.