This caution is for the ones who make their movements coincide with the movements of the imām, whereas the sunnah is to wait until the imām completes his movement. As for the ones who precede the imām in his movements – specifically when raising their heads before the imām in the rukū’ or the sujūd – then they fall under the threat of the Messenger of Allāh (ﷺ) who said:
“Does one of you not fear that if he raises his head before the imām that Allāh will make his head the head of a donkey, or Allāh will make his likeness the likeness of a donkey?”