When the whole fuzz about gender discrimination started, Microsoft and Google published numbers, claiming women got the same pay at the same positions as men. Knowing there's discrimination from personal experience/feeling, I theorized, that women are discriminated in a different way: they don't receive promotions.
Under otherwise similar circumstances having children does not feel to be enough to explain why of 100 women hired in tech on professional roles less are promoted to higher positions, than of 100 men. That trend is (at least anecdotally for me) observable even before people become parents.
This "Bamboo Ceiling" shows the same effect for another potentially discriminated group of people.
The neat thing about this form of discrimination is that you can claim to be fixing "the pipeline" all you want and you can still maintain the discrimination, because the leak is after the pipeline. The dominant group isn't threatened by competition if they fund efforts to increase the number of underrepresented groups in grade school or college STEM education, as long as those college graduates aren't later competing for senior jobs on a level playing field.
You can try strawman arguments like "they are not equally qualified" or "reverse sexism" or "they are doing it to themselves by not negotiating", but a cursory look at any reproducible social sciences review disproves those (laziness to not use google or google scholar is a tiresome excuse).
The parent comment to yours was poorly worded and snarky, so you have a right to be upset. But still, I think your reasoning is flawed. People are generally promoted by their competence and their negotiating/office politics skills, and you can't claim that those are the same across all genders. Why would women, who are fundamentally different than men, have the exact same competence and negotiating abilities as men? There's no reason the two genders should be equal.
If you really have two equally skilled engineers, one male, one female, and only the male is promoted, that's sexism. But two engineers are never the same, so you can't make that argument.
But say I'm doing a project in Java and they are about equal, I keep giving Al the meaty work then use it justify a promotion, which I can't justify for Marcy. It's not that she's that much worse, I just never gave her the chance to prove it (blah blah peter principle, perform at next level, etc...). That just might be a bit sexist.