That is not true at all. You can measure testosterone. You can measure androgen sensitivity. You can then probably work out where on the 0-12% advantage scale someone is and set a level that is allowable to compete. The argument that raised testosterone is automatically a huge advantage is lazy, unscientific and incorrect.
Except, it is, testosterone and other male hormones don't need to exist in the body at elevated levels at the time of competition to afford significant athletic advantages.
Most professional athletes who dope, "cycle", cycling means that you go on the gear, make gains, get off the gear, lost some/most of the gains, but most importantly KEEP some of those gains.
We're also ignoring the foundational and structural advantages going through puberty with male hormones/elevated testosterone grants. Now this isn't about any one particular person, this is just a general truth. Your hormones inform how your body develops. If I had transitioned to female at 20 years old, I'd still be 6'4, I'd lose some lean muscle mass, my shoulders delt to delt might go to 22 inches from 24 inches. I'd still have a larger heart pound for pound, larger lungs, pound for pound, denser bones (though they'd become less dense than they were), I'd have larger and more robust insertion points for my muscles, tendons and ligaments. I'd have male bone structure which is proven to be more efficient bio-mechanically for most tasks.
My body would be running on female hormones, but it would be a body built by male hormones, and that itself affords a measurable advantage.
Removing the current testosterone from a trans athlete isn't the whole story. There are hundreds of thousands, if not millions of men who are as big, if not bigger than I am, and combine some degree of athleticism that I had in my youth. There are dozens of women, maybe a few hundred, who could compete with that combination of size and athleticism. How do you account for that? Are we just going to ignore biology?
Simply put, there is more to the story than maximal output afforded at a specific elevated testosterone level at any given point in time. More test = work harder, more fitness, more confidence, go off the test, lower the test whatever, you still keep some of those gains. That's why doping exists.