There's less pronation, or perhaps it is better to say that the timing of the pronation is different on a topspin serve. The pros usually turn more away from the court and the racket path is more across than toward the target. The racket still lags meaning that pronation (internal shoulder rotation) is need to turn the racket face to the ball, but it occurs later in the swing because the shoulder usually aren't turned as much forward. Topspin is also due to impacting the ball at a slightly lower level and more to the left.
The topspin serve needs a horizontal component like any other serve and the pronation (internal shoulder rotation) is what adds a lot of that juice.