This BTW is the first time you've articulated a principle to distinguish the two. "Financial".
First, the retort is morally weak. You are basically saying I want a fair playing field but I don't care enough about one to be inconvenienced or to have to pay for one. That leads to the inference that you don't really care about a fair playing field and all your protestations are really just pearl clutching. The only way this gets you there in a logically consistent manner is if you consider the trans thing such a greater or equivalent moral wrong to outright cheating, that it is morally worthy of a stricter structure.
Second, from a strictly utilitarian point of view, we know it's not true because they do manage to do it on the college level. If ECNL and MLS Next soccer are really so important, we have a guidepost for how it would work.
Third, given California issues birth certificates to trans kids now on demand, you have the same utilitarian problem with the trans individual. You would need to establish a protocol to test and hunt down anyone suspected of being trans, which again carriers farther than we do for PEDS. You have not articulated what that protocol would be or how it would be effected, because playing with a birth certificate issued by a state government would legally immunize you otherwise.
Fourth, as in the case of the Utah girl previously discussed, such as system, short of universal testing, sets up the externality that a cis girl can be innocently accused and subject to testing for being trans. Unlike the Utah situation, since school records are not an option, such an examination would be much more intrusive.