I think that the following halochos indicate that there is no bittul of the Rabbinic tumah of liquids.
Rambam Hilchos Shaar Avos Hatumah Chap 7 says:
Halocho 1: (Food that contracted impurity does not impart impurity to other foods according to Scriptural Law.) Nor do liquids that contracted impurity impart impurity to other entities according to Scriptural Law. Nevertheless, according to Rabbinic Law, it was decreed that impure food that touched other foods would impart impurity to them. Similarly, if impure foods touched liquids, the liquids are impure.
With regard to liquids, by contrast, whether a liquid touches a primary source of impurity, a primary derivative or a secondary derivative, that liquid is considered as a primary derivative. It imparts impurity to another liquid, which in turn can impart impurity to another liquid, even if there is a chain of a hundred. For there is no concept of degrees of impurity with regard to liquids.