From that Stonewall piece.
"The definition of sex -over medicalised language and should include that it is assigned at birth"
Why woulld you take advice from any organisation which uses incorrect terminology? It should be "observed" not "assigned".
"There is no direct and consistent research to suggest that transgender female individuals (and transgender male individuals) have an athletic advantage in sportand, therefore, the majority of competitive sport policies are discriminatory against this population"
Again this is untrue. There is plenty of scientific evidence, that male bodied people have a distinct advantage when competing in the female category.