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.