I think one would be hard-pressed to find anyone who claims there are no significant differences (at some point, the question reduces to one of definition).

The argument is over what those differences are, how much they matter (i.e. a 1% difference in the mean? Who cares), and how malleable they are given modern medical science.

(And that's before one gets on the topic of whether those differences should imply a societal structure that pigeon-holes people based on their biological sex).

I’ve never met anyone who has said this, and I’ve worked in quite left wing spaces for a long time. I believe you may have fallen for some caricature or other of the left. I’ve no doubt someone has said that, but I think you’d find that belief in bigfoot or flat earth is more common, because it’s quite apparent to most that there are indeed differences between sexes. I think what tends to trip people up are what I call “dude looks like a lady” situations where people, either through accident of birth or other means, appear androgynous; it seems gender is more of a bimodal distribution rather than a strict binary. Again, this seems rather apparent to me just having met some people who don’t exactly fit the mold, but apparently this is also something of a political controversy.