Shiryu has eaten the Suke Suke no Mi, a Paramecia-class Devil Fruit which gives him the ability to turn himself and whatever he is holding invisible. He acquired this fruit soon after his crew murdered its previous user, Absalom.
It’s a very useful devil fruit for sure as it can conceal the user and anything he is in contact with. However, for New World standards, my first impression was disappointment because while obviously it’s hard to fight someone you can’t actually see, especially a skilled swordsman such as Shiryu, the whole point of Observation Haki is to feel the individual’s presence and every movement even while not seeing it.
Surely Shiryu has an advantage against lower level enemies, but Observation Haki users should remain unfazed by his invisibility.
So, let’s just assume Suke Suke no Mi will not be affected by Observation Haki otherwise this fruit is just useless. What if Zoro is the only one in the world who can sense Shiryu? Not with his Observation Haki but with his sense for cursed things. We saw him more than once to sense cursed swords and even know where exactly they are positioned without actually seeing them.
So if Shiryu has a cursed sense himself or simply has a cursed sword (remember that 3 Kitetsu swords are not the only cursed swords) Zoro would be able to sense the invisible sword coming at him without actually seeing it.
*Theory by minor25