Exactly, it seems people don't comprehend the meaning of the word 'overpower.'
Now getting back to who is more powerful against darksiders, yes, Mace has an extreme talent with Shatterpoint and mastery of Vaapad, and yes, those are two advantages he possesses that Yoda doesn't, however nobody here can quantify the advantages that they offer against darksiders and claim that the advantages are to such a degree that they make Mace a bigger threat against darksiders than Yoda, despite Yoda being more powerful.
Now let's go back to the quote in the RotS novelisation - 'most powerful foe that the darkness has ever known.' In this context, 'most powerful' actually means 'most powerful against darksiders,' as in respect to the story, how powerful Yoda is against non-darksiders is completely irrelevant, as they are not his enemies, and he would never be facing them. When somebody is labelled as 'most powerful' in such a context, it mean 'most powerful against his enemies,' it's that simple.
So to go back to my earlier point, despite those two advantages that Mace possesses, Yoda is still the more powerful against his enemies than Mace is. So again, the fact that Palpatine's skill and ferocity increased by such a substantial degree makes it pretty obvious that he was going easy on Mace.