Skullcrown does an Unkillable with her own Passive.
Most people who fight against a Skullcrown are aware of the Skullcrown Passive Ability to put up Unkillable.
Thus, They will have in there possession the necessary resources to handle the situation.
They will anticipate the Unkillable Buff and have someone to remove it or ignore it.
It will defeat the purpose of using the Swift Parry Set.
You are better off using Swift Parry on a hero which doesn't have Unkillable Passive.
It will catch the enemy by total surprise.
Just to give you an example.
I use Rotos vs. Skullcrown.
Rotos A3 ignores all Unkillable Buffs.
Obviously, It will not matter if your Skullcrown Passive triggers or Swift Parry set triggers.
I will be using Rotos which will counter both.
Lets give another example:
Lets say you are using another damage dealer like Trunda.
Well against Trunda - I like to use my Foli or Brakkus.
Than if your Trunda is in Swift Parry Set.
It would come to me as a total shock because I didn't anticipate your Trunda with Unkillable Buff.
My Foli or Brakkus wouldn't be able to bypass your Unkillable.
Than your Trunda may survive and do big nuke on me killing my team.
It would be a horrible turn of events for me.
That is the sort of thing which you should consider when using your Swift Parry set.
That is why I think using Swift Parry set on Skullcrown is bad idea.