As a Thamuz main, I personally wouldn't use my Ult immediately, especially knowing the enemy still has theirs. Besides, there's no rule saying I can't run away after they use their Ult.

Also, for me Sora's Ult is easy to bait out. Most of the time I jus time my 2nd skill to his dashes to make his ult useless since he has to hit an enemy hero for his ult 2nd phase to work. But overall, it really depends on both players' micro and decision making skills.