Specifically the downplaying of biological differences between men and women and the belief that any and all differences between outcomes for men and women are down to patriarchal socialisation and literally nothing else.

Either you've only been exposed to cherry picked examples of people claiming to be feminists who say these things, or you're being intentionally reductive.

Feminism does not downplay biological differences between men and women. Feminists are the main ones highlighting our differences in size and strength in order to protect female only sports, changing rooms, rape shelters, and prisons.

If you mean neurological differences specifically then yes, feminism objects to that. Because there hasn't been any good research to show sex differences in brains apart from grey matter density and overall size. Nothing has suggested one sex is more or less intelligent, more or less emotional, etc.

The main solid piece of evidence that has been found is proof of innate sexual orientation. News headlines misrepresent the results by saying things like 'studies show gay men's brains are similar to women.' When a more accurate statement would be 'brain similarities found in all male attracted individuals.'

Assumptions that male and female brains are innately different structurally and innately function differently are what lead to autism and ADHD continuing to go undiagnosed in women, because socialization causes those disorders to present differently in women. Assumptions like that also lead to many women being forgotten in the history of mathematics and engineering, because people still think women can't be as smart as men, and that it's a """biological difference"""" between the sexes.

Even if we were to discover that on average men are Y and women are X as far as brains are concerned, that would not be an excuse to claim that outliers in either category make them the opposite sex. Or to claim that those averages hold true for all men and women.

Feminists object to ideas that men are somehow more rational and more in control of their emotions, while some men, in their own words, claim that they need to watch porn to stop themselves from raping a woman, or that the reason teenage girls are sent home for tank tops and shorts is because teenage boys can't control themselves.

Huge difference between that and 'literally all differences are due to socialization.'