We usually refer to a potion in the sense of a particular mixture made up by someone, rather than a generic substance like milk or wine. So you could say 'The witch drank some of the potion she had made'. (Of course, we can also say a wine to refer to a particular kind.)
It's possible that some of the quotations found by Google are using 'some potion' to mean 'an unspecified potion' rather than 'a small quantity'. In some cases, it's part of a phrase like 'some potion ingredients' or 'some potion vials' (ingredients or containers for a potion) - or it can mean 'some (of the previously mentioned) potion'. So it's not correct that we cannot say 'he drank some potion', but the word is usually thought of as countable.