No, "this" in the final sentence is not taking the place of the phrase "... to hear it asserted that the inequality of rights between men and women has no other source than the law of the strongest."
In your final sentence, "this" is the subject of the sentence:
"This (subject) is (verb) what makes it so strange . . . to hear it asserted that . . . [the assertion] (object of the verb "is")"
The antecedent of "this" in your final sentence could be any one of the following:
- The entire preceding sentence.
- The entire first sentence.
- Both of the above.
In short, "this" in your example refers to the writer's argument itself.
Why? Because it is much easier to use "this" instead of referring back to the fact that "The only such presumption which it could be supposed to have, [sic] must be grounded on its having lasted till now, when so many other things which came down from the same odious source have been done away with."
See, for example, Wikipedia's article on antecedents, and particularly, example (i).
Here's a simpler example:
In English, it is very common to encapsulate large ideas with a single demonstrative pronoun, for brevity. This can save you a lot of typing.
In the above example, "this" plays the same role it does as in your question, just on a smaller scale.