It's a matter of PUBLIC RECORD already.
My inclination straight away is to agree with you due to transparency and in particular records of the existence of prisoners ought to be public so that governments do not create secret torture prisons. However, in this case, there are a couple of nuances. First, if your assertion that it is a public record already were true, then all the requestor would need to do is a manual count of those public records. Since that hasn't been done, there appears to be something more to it. Perhaps you could comment on what is different? That brings me to the second point, ought the public have a right to know which persons born male are now female or in transition or have had particular medical interventions? I think the answer to that is no. From a legal standpoint there are also some privacy laws such as HIPAA and so forth that block the particulars of knowledge of specific people but there are also discrimination laws that relate to privacy as well. And I don't even know what else. So, third, at work, they'd publish to public info various statistics, like gender and race breakdowns of salary and whatever. However, when the occupation had like 1 or 2 people or the race had like 1 or 2 people, they'd not report it because then it would risk that the public could infer which individual received what salary or benefit or whatever. And it is plausible that the ACLU is blocking this for a related reason that there are so few persons in this class that reporting whatever has been requested violates privacy. Now you said it's already public, but I am thinking that the entirety of the request probably isn't. Otherwise, the ACLU would not have this stance. Perhaps you could provide a link to the ACLU's website and their stance on this issue so we can have a reputable source rather than just chatter and my reasonable speculations about it?