רב שך means to answer why we do not learn from סוטה [laws of סוטה פרק א ] to say even ספק טומאה  ברשות הרבים טהור אפילו כנגד חזקת טומאה. I asked on his answer a few days ago and it occurred to me today that my question was based on a misunderstanding of what רב שך was saying. What he is saying is this. If we would learn from a סוטה to a רשות הרבים then in fact even ספק טומאה בשביל things that have a חזקת טומאה would be טהור. It would be because of גזירת הכתוב. But in fact we don't. We only learn from סוטה in a  רשות היחיד that a doubt is considered to  be definite.] And THIS explains the גמרא that otherwise is hard to understand. The גמרא says the חכמים learn from סוטה to say a doubt of טומאה in a רשות היחיד is definitely טמא. Then it asks, then why not learn from סוטה in a רשות הרבים. Answer: the מקוה is not like a סוטה. The מקוה is a doubt because of the volume. The question about volume is not connected with a private or רשות הרבים. The doubt about the סוטה is because of being alone with a man in a  רשות היחיד. There is no סתירה in a רשות הרבים. Then the גמרא asks but even so is it not so that every doubt about טומאה  a רשות הרבים is pure? Answer: it is a case of two things pushing towards uncleanliness. The חזקת טמא of an object עד it has been made clean, or the person that is handling  the object. Plus the חזקת השתא of the מקוה which was measured and found to be lacking ארבעים סאה. This גמרא is very hard to understand. At first it says the reason the doubt for the מקוה is unclean is because we learn from סוטה. Then it seems to change its mind and says  the reason is because two חזקות against one. Now however we understand the גמרא perfectly because it is not learning from סוטה about anything in a רשות הרבים. Only the  רשות היחיד. As for the רשות הרבים we learn nothing from סוטה at all. Things are clean in a רשות הרבים and that would include סוטה if they have no prior חזקה at all. Not טהור or unclean. But if anything would have a חזקה of being unclean it would be טמא.