Jep, that's the difference. Zoro has to train. All the time. He had to train to tame Enma and now he can unleash his Haki through Enma. PU would mean that Enma made Zoro or his Haki stronger as soon as he took the blade in his hand. Which is not the case.
Zoro said: "So as soon as I got used to this blade, this is how much stronger I became".
So Zoro basically never said Enma makes him stronger, but that him mastering Enma is a testament of him having become stronger.