Question: why is mui goku so strong?
Answer: It's because, during his fight with Beerus, he absorbed the power of SSG into his base form. Pretty much non-debatable, as Goku himself didn't even notice his own power decrease, and was now able to connect punches with Beerus while still in base even though a few hours earlier, Goku SS3 was just a yawn for Beerus.