I would give that study a mixed-to-poor review, to be honest.
The study - which presents no new scientific data, it is only an essay and the authors are philosophers, not medics - makes several admissions along these lines:
Knox, et al. wrote:[...] Given the absence of evidence directly related to
elite transwomen athletes, and as transwomen were previously
biologically male, we extrapolate from evidence based on male
physiology.
[...]
Extrapolating from this it is plausible that transwomen retain
some of that advantage. We acknowledge that these findings
rest on extrapolations from male physiology, and encourage
further specific scientific investigation regarding elite trans-
women athletes.
i.e. Their conclusions are predicated on the
assumption that trans women retain the advantages of male physiology, they don't
actually know. They later go on to say things like,
Knox, et al. wrote:Combined, these three factors (cis-women cannot attain the
advantage, all-purpose benefit and magnitude of the advantage)
provide a strong argument that transwomen have an intolerable
advantage over cis-women. [...]
But they haven't demonstrated an advantage, let alone the magnitude of said speculated advantage, so it's navel-gazing, isn't it?
This is enough to sink the premise of the essay in my view. Another issue I was found was the exclusion of cis intersex women from the study:
Knox, et al. wrote: The conflation of transgender and intersex people
leads to confusion around effective testosterone versus
testosterone levels. This article solely discusses trans-
women, and therefore our arguments are not compli-
cated by androgen insensitivities in which receptors
are not sensitive, or only partially sensitive to testos-
terone.
But this (perhaps wilfully?) ignores the point that some intersex cis women do have high effective testosterone, and that there are sports in which IOC guidelines aimed at trans women have only ever affected intersex cis women (e.g. Caster Semenya in middle distance running).
They also make a point about "intolerable unfairness",
Knox, et all. wrote:It is not entirely clear how to distinguish between a tolerable
and an intolerable unfairness. [...]
This suggests that an advantage is unfair if no member of the
category (eg, cis-women) can attain that advantage. [1] [...]
Another way to distinguish between tolerable and intolerable
advantages is to consider whether the property under consider-
ation provides an all-purpose benefit. [2]
Both points [1] and [2] are also true for variant ACTN3 genes, but no-one is considering subdividing sports categories into ACTN3-variant people (with an inherent, unattainable advantage) from the wider ACTN3-wildtype population. It becomes difficult to justify this concept of "intolerable unfairness" with such a glaring exception.
A minor point is that they use a misleading and contradictory definition of "trans woman", first restricting the scope of their argument to pre- or non-orchiectomy trans women, but later arguing all trans women are advantaged. I think if you restrict the scope of your study you should make that clear and stick to it in your arguments and conclusions.
Anyway, here is their proposed solution (I emphasise they have not fully justified or defined the problem it purports to solve):
Knox, et al. wrote:[...] there
would be multiple divisions rather than simply male and female.
Second, based on the results of the algorithm, athletes would be
placed into a division which best mitigates unfair physical and
social parameters.
The physiological parameters could include
► Size, for example, height and weight.
► Haemoglobin levels.
► Maximal oxygen uptake (VO2 max).
► Whether the athlete transitioned before, during or well after
puberty.
► Past and present testosterone levels, and the presence of
testes.
► Previous characteristics of physiology that are not changed
via hormone therapy, that is, bone strength or structure,
lung capacity and heart size.
The algorithm would need to be tailored to specific sports as
a physiological factor will be more advantageous in some sports
than others. For example, weight would need to be accounted
for in weightlifting, and perhaps wingspan for swimming.
So they want to end the "gender binary" division in sport by creating multiple categories that clusters entrants based on their overall physical characteristics. The spirit of this is probably the first thing I liked in the entire paper and is preventing me from panning it entirely.
I don't like, however, that their justification for the chosen parameters are "these are characteristics that might not be changed during transition". They would develop a much more holistic view of fairness and inclusion if they undertook a broader review of variance and advantage in sport. I believe better advice for the elite sporting community would simply be "figure out the parameters associated with performance in your sport, then cluster your competitors based on those (implicitly: regardless of their gender or intersex status)".