The argument is not about that.
It's that "Those very sexist 60s/70s fields still had a lot of women going in, more than women in IT today. If women could go into those very sexist fields then, it's not sexism that keeps them out of IT today".
If so, I can't see where we substantially disagree. As I've repeatedly said, I make no claims about why the gender balance shifted in law and medicine in the 60s. You appeared to suggest that this had nothing to do with levels of sexism. I'm skeptical of that claim (broader societal attitudes are important too, not just the sexism level of lawyers and doctors vs. the sexism level of scientists and engineers), but it's not as if either of us has actually researched this in detail.
The key point is that tech now is disproportionately sexist, as you appear to acknowledge.
In the sense that if sexism is the major force that prevented women from going into IT, then the much higher sexism of doctors and lawyers in the 60s and 70s would have prevented women from going into those fields too. But women were far more in those fields then, than they are in IT now.