independent from the kind and quality of the used reference cloc
Independent from the quality of the clock, not independent from the external clock itself.
But it is a semi proven fact (semi because not tested with all combinations, but only some) that no device improves its jitter when clocked externally, no matter how much better the external clock is. It will either worsen when clocked externally (cheap or bad design) or at best stay the same (best and more expensive design). And that is what RME says, this device will have just as good jitter result when clocked internally and externally.
Or are you confusing reclocking and SRC (sample rate conversion) at same sample rate?
HDSP9652+ADI-8AE 2X HDSP9632