the phrase "science proper" [duplicate]

The Collins Learner's Dictionary explains this well.

4. adjective [n ADJ]
   You can add proper after a word to indicate that you are referring to the central and most important part of a place, event, or object and want to distinguish it from other things that are not regarded as being important or central to it.
   A distinction must be made between archaeology proper and science-based archaeology.

So it does indeed mean something like genuine science.

There is a big difference in implication, however. If you say genuine science, you are subtly implying that the things you aren't including are fake science, whereas if you say science proper, what you are implying is that the things you're not including are on the outskirts of science, which is much less denigrating of these non-included things.

Proper science might mean this, but it also might mean any of the other definitions of proper, while science proper can have only this definition.

Einstein is using "science" here -- "science proper" -- in what etymonline describes as its "modern (restricted) sense":

Modern (restricted) sense of "body of regular or methodical observations or propositions concerning a particular subject or speculation" is attested from 1725.

"Though shalt not lie" is normative or prescriptive, i.e. what one should ideally not do. Some social sciences, e.g. those dealing with morality might work with normative ideals which have no basis in experimental observation. Psychoanalysis would also not count as science in this restrictive sense since the theories are not falsifiable.