The video you linked to says that in the movie it's explained that Oppenheimer spoke German, and learned Dutch in a short amount of time. If so, him speaking Dutch with a German accent seems appropriate, if slightly exaggerated.
However a novice in a foreign language will speak more slowly than natives, not more quickly, even if reading a prepared speech (as was likely the case in that setting, and especially given the advanced language used that no beginner would have mastered), so not sure what the thinking was there.
However a novice in a foreign language will speak more slowly than natives, not more quickly, even if reading a prepared speech (as was likely the case in that setting, and especially given the advanced language used that no beginner would have mastered), so not sure what the thinking was there.