I can agree somewhat, but you missed few things
- While Zoro doesn't have the speed to keep up with Doflamingo, he has the reaction and swordsmanship to do that. Law could hold his ground a little and Zoro is a better swordsman than him with better angles cover(3swords)
- Doflamingo still would have a hard time putting Zoro down. Zoro is an incredible tank(even pretimeskip, so let alone post-timeskip) and possess strong armament haki to boot and we see that it can be used to block his string ability(Law managed once) or even if the attack bypass the armor, it reduces damage considerably.
On the other side, Zoro possess strong attack power(it one shotted Doflamingo's strongest subordinate without Ashura), so if Zoro were to land hits, it would be ugly. Yes i know Doflamingo is an incredible tank himself for the reasons you presented, but he won't be unphased by those attacks either. In the end then, Doflamingo can't finish Zoro quickly. It would require him mid difficulty to put Zoro down with a combination of his superior agility, trickery and other string abilities.
By the time, Luffy himself can easily decide to dispatch Vergo while in gear4(the time Doflamingo will want to deal with Zoro) and thus quickly making the fight in a 2vs1 against in Luffy-Zoro favor