To address the issue raised in the 1st link about "gender assumptions," what some feminists (especially those who aren't athletes) don't want to admit is that gender testing exists to protect women's sports and the great majority of women athletes.
Sports are segregated, unlike almost everything else these days, because the best women can't compete with the best men. If there were only unisex sports teams, almost no women would have the opportunity to compete.
If, for example, a man could choose to apply to be a scholarship athlete on a men's or women's college track teams, there are plenty of men who are faster than most women and would be willing to run on the "women's" track team in return for a free college education. So you have to segregate to preserve women's sports.
Given the various permutations in biology, there are difficult cases where some people may have some of the typical physical characteristics of both men and women. But you still have to draw the line somewhere in order to protect women's sports. If there is no line, it hurts women's sports.
I have no idea about Brittney Griner in particular, but there is nothing wrong with having a requirement that you be a biological woman according to a particular definition in order to be eligible to compete in women's sports. You can argue about what that definition should be or where the line should be drawn, but there has to be a line somewhere.