This is a very recent video - 6th October (4 days ago):
His build: Swordsman -> Highlander c3 (proof: skull swing) -> Corsair c2 (proof: Subweapon cancel)
His stats: Almost full dex (shown at 1:10 in video) - note his evasion is quite high and he barely gets hit in this video.
As you can see, in the first 10 seconds or so he has the Subweapon Cancel buff on but his attack speed wasn’t much different from the rest of the video. Certainly very different to the old corsair here:
Also I believe Skull Swing lowers a set amount of defense regardless if the enemy is wearing a shield or not. I think that part about shield in the skill description is just due to the translation from Korean, don’t take it too literally.
PS: Highlander actually works well with Corsair because Highlanders can lower defence, which helps Corsair’s auto attacks a lot since the attack from 1H weapons are weaker than 2H.
It looks like subweapon cancel just adds an extra attack - if you see the first 10 seconds, every 2nd swing seems to pop out 1 extra number. Apart from that I can’t really make out what Subweapon Cancel does anymore…