Also, Yoru is not made strongest. Yoru was confirmed strongest.
Yoru was forged into a black blade, Mihawk said that any blade can become a black blade. Ryuma forged Shusui into a black blade through countless battles. The rank/grade of a sword can be raised when it is forged into a black blade according to Hitetsu. So Shusui was probably one of the 50 grade swords and then got upgraded to 21 great grade swords when it was forged into a black blade.
So either we say that Mihawk just stole/obtained Yoru from someone who made Yoru the strongest sword (black) or Mihawk got Yoru and then forged it into a black blade and made it the strongest blade in the world. There are 11 other supreme grade swords with Roger and WB's blades being 2 of the 11.
And if we apply the grade upgradation then either Yoru was one of the 21 great grade swords and then became a supreme grade sword when it was forged into a black blade or Yoru was already one of the 12 supreme grade swords and now it is the strongest because it is the only supreme grade sword which has been forged into a black blade and is above the other 11.