Depends on the activist, I suppose, but sure, people are gonna misuse science.
By the same token, people insisting that sex cannot be understood as anything but a binary are giving oxygen to the sex-is-anything-I-want-it-to-be crowd, by sticking to a bad talking point. It would be better to say, "Sure, you can, if you want to, understand sex as a distribution, but it will be close enough to binary that it's not clear what you gain by doing so."
I still maintain that this focus on the word "sex" strictly and solely in its biological sense is misleading
a propos the transgender identity debate.
And that's because there are two widespread reasonable definitions of "sex". One is indeed the strict biological definition. But the other is effectively a proxy for "gender". People have become used to "sex" and "gender" being effectively synonymous; but now that medical science has decoupled the two terms, this isn't true any longer. However (for obvious and understandable reasons), many people continue to use the word "sex" as a proxy for "gender".
So, in fact, the word "sex" - in the context of a debate about transgender identity - is a lot more complicated than simply "biological sex". Only a few fringe activists, for example, believe that when people transition, they transition their biological sex. Most pro-trans-rights people know full well that transgender people retain the biological sex they had from birth, and that it's purely their gender which has transitioned.
This confusion over definitions of terms means that most legislatures - rightly - consider that in certain contexts "sex" can be a weapon against transgender people. For example, if someone harassed a trans man by saying "We all know what sex you really are - you're just pretending to be the opposite sex", this would most likely be considered aggravating hate speech: the speaker here is using "sex" as a proxy for "gender".
The confusion extends to the use of "female" and "male". In a similar way, up until recently, those two words have been wholly synonymous (in their noun form) with "woman" and "man" (and in their adjective form with "woman-like" and "man-like"). And many people still use those words in that way.
I go back to one example of this confusion in action: the part on the identity page in English-language passports which asks for the person's "sex" - where the two possible answers are "male" or "female". Again, this Q&A is something of a relic from a time when sex and gender were entirely synonymous. More and more legislatures are now - appropriately and correctly - allowing transgender people to adjust this field to their trans gender, even though the label is "sex" and the two possible responses are "male" and "female". In doing so, these legislatures are sensible & rightly considering that the question (in this context) pertains to gender identity more than biological sex - that, in other words, the word "sex" in this context does not mean "biological sex" but is instead being used as a synonym for "gender".