I think you are correct to some extent. That part of the reason why penholders do so well against Europeans is because they aren't used to playing them. But in another respect, they do badly simply because penholders are the masters of the short game. And when you control the short game, you control who attacks first. It's why Ma Long loses to Wang Hao, while Ma Lin doesn't. Ma Long's short game isn't up to par with the penholders therefore because he can't attack first, he has to react to Wang Hao's first loop, which more often than not, is too high quality even for Ma Long's counter looping style of play.
Boll has played penholders plenty of times in the past, not only has he played in the super league in previous years, but there are players like Wang Jianjun who play in the Euro leagues. Wang Jianjun even played either on the same team as Boll or practices in the same gym with him often, because I have seen several videos of them in the same gym. So I don't think its necessarily so much that Boll isn't used to penhold, but just that, his short game, is even worse than Ma Longs.