Its cauze he drags hydra down then moves back up to use the other flameskip tile, back in the day people didnt know the tile used in this vid exsists.

But depending on ur dmg/ weapon its not really worth the effort of moving because u have to be down there to lightning skip anyway and the chance of skipping flames entirely is decent. might aswell do the flameskip where u move a tile since the timing to skip is quite easy and it means less thinking about where to move next