The null hypothesis is that males have an advantage. Trans athletes were allowed in women’s competition with no evidence that hormone treatment removes this advantage. Until there is evidence that the advantages of androgenisation are removed or wholly mitigated by treatment the rules are just plain wrong.
It might be the "default" assumption that males have an advantage. But "null hypothesis" means there is no relationship between two variables.
And from empirical standpoint, you cannot support a null hypothesis, you can only reject it. (How do you prove that something does not exist?)
So the burden of proof is on the side that claims that the relationship exists between two variables. In this case, we need to prove that trans athletes have residual advantage even after hormone treatment. And the only way to prove that is by letting them compete with hormone treatment and compare before and after performances. Ideally, this "experiment" should be conducted in a situation that does not hurt the third party (mixed gender open competition, for example). But the current rule was written without having enough data to justify it. And in order to reverse the rule, you need sufficient evidence. Someone like Eastwood competing with hormone treatment gives us some of those data points to change the rule, before a more talented trans athlete might come around to compete at the elite level.
This is not a perfect solution, but we don't live in a perfect society.