First of all, yes, swords in general are power ups, if you transition to a blade of higher quality. That became clear, once swords were divided into classes; into normal weapon and Meito, and Meito having 4 classes themselves (grade, skillful grade, great grade, supreme grade). A supreme grade sword is obviously much stronger than a grade sword. But that power up does not always come without the effort of mastering that blade. In both Kitetsu's and Shusui's case, Zoro had to get used to it.
Now regarding Enma. Big Mom called Enma "no ordinary Katana". That in itself does not mean, that the power of Zoro's strikes comes from the blade itself. Zoro also pointed out, that Enma has a special aura, which he could sense without even seeing or touching the sword. Tengu also drew parallels between Kitetsu and Enma, saying that both are swords that cannot be wielded by the weak. So Big Mom feeling the aura of Enma in itself doesn't take away feats from Zoro.
Kaido feeling Oden's presence is also no proof of the blade holding special powers, as we have learned, that the blade inherits the will of the user. But the story taught us, that blades are no exception in that regard. People can also inherit someone's will, like the Scabbard's inherited Oden's will. Even Devil Fruits can inherit the will of the former user, as the Mera Mera no Mi inherited Ace' will and Sabo, upon burning Burgess', said, that although he wanted to let him go, it seemed like Ace did not forgive him.
Now what's Enma's power by definition? Drawing excess amounts of the
users Ryuo. That's right. The attack Zoro used, was amplified with his own Haki, and he used a technique that he used already pre TS against Ryuma. There is no proof whatsoever, that the blade
stores it's users Haki, let alone over the span of 20 years. This is why Kaido only commented on Enma having Oden's presence, while in the case of the scabbard's, he questioned whether they are using Oden's Ryuo. Had he said the same about Enma or Zoro, that it is Oden's Ryuo, it would have been another case, but that case did not happen.
Lastly, some appreciation of Zoro from different people, including including Big Mom:
- Tengu said that Oden, so far, was the first one to tame Enma; no one before or after him ever managed to do it. Kinemon, a Scabbard, also said, that he would refuse to take it. Yet Zoro took and (mostly) tamed that sword, which speaks for his incredible Haki, will power and skill as a swordman. Tengu however goes even a step further and says, that Zoro will eventually achieve what Oden couldn't: forging Enma into a black blade
- Big Mom told Kaido, that he underestimated Zoro. So she talked about him as an individual, rhather than complimenting the sword itself.