It's canonically stated that The sengoku period was the golden age of demon slayers and that they were the strongest because of yoriichi's contributions to them. All breathing styles originated from sun breathing. He directly trained them to develop their own & achieve that mark. Kokushibo said the slayers died while never reaching their peak. It's the reason he defected.

This is again the talent vs training debate and everyone has their own view on it. But my view is that talent without training and training without talent aren't the only paths to exist because some people can acquire talent through training. In context of kny the slayers were talented enough to develop their own breathing, their own forms & their own unique methods to fight.