As I understand it, Elephant, the historical origins of the rules on testosterone are more complicated than that. The levels were based on a survey of (IIRC) 2000 female Olympians, and were set to make sure women with PCOS who have slightly raised testosterone levels could continue to compete. Women without PCOS typically have levels of about 1 to 1.5 nmol/litre, women with PCOS more like 2 to 2.5. They then added 5 standard deviations to the distribution (the level CERN use to establish the existence of a new subatomic particle) to make sure outliers weren't excluded, bringing the limit up to 10nmol/litre. Men have levels between 10 and 30 nmol/litre (with the majority of men falling between 20 and 30 nmol/litre). Caveat - quoting these figures from memory - I know the 10 nmol /litre cut off is correct though.
This ruling meant that intersex women had to take androgen blockers to bring their levels down to the Olympic standards. (It's widely suspected - though without confirmation, because her medical records are, quite correctly, confidential - that this explains why Castor Semenya's initially brilliant performances fell off to near-average for a world-class runner while this rule was enforced, then suddenly went up again this summer when the rules were relaxed).
Now we come to the complicating factor. Intersex women can suffer from partial androgen insensitivity (PAS), where their bodies produce lots of androgens and some androgen receptors, and they are partially able to use these to build up musculature in excess of XX chromosomed women, albeit not to the same level as men with the same levels of androgens. Or they may suffer from complete androgen insensitivity (CAS) where they have male levels of androgens but no androgen receptors, so retain XX patterns of musculature. It is thought that Duttee Chand suffers from CAS - she successfully brought an appeal against the IOC on their enforcement of the use of androgen blockers, leading to the upper level of 10nmol/litre being relaxed. The result appears to have (note, appears - the medical records are not in the public domain) led directly to the top three medal spots in the Rio women's 800m final going to intersex women.
Note that this shouldn't have anything to do with the issue of trans sportspeople, but (as always) intersex issues have been hijacked by the extreme transactivists to argue that in effect individuals with biologically male physiology should be allowed to compete in women's events.
But, as I said upthread, other than the stupid wording of "mixed gender", what's being proposed has nothing to do with this - it's mixed sex team events, which are not intrinsically unfair, and can in fact be fantastic sports (e.g. korfball, ultimate frisby).