This is extremely subjective and can be a lot of things.
There is nothing subjective about it.
- We know that Enma is a "cursed" sword.
- We know that "cursed" swords have special auras.
- Zoro was already shown to be able to sense Sandai Kitetsu's curse even without seeing it in Water Seven.
- Then it is perfectly normal that Kaido who already fought against Enma recognizes her, because of her aura.
- It also justifies that Big Mom feels something strange, the "curse".
It is that everything is perfectly explained, there is not a single loose end. It is 1000 times better than Oden's Haki explanation which has 5-6 major gaps.
Between something known that explains everything and something unknown that has many gaps, I think it is obvious where the subjectivity lies.