That study examined the Patient assessment reports created between August 1st 2016 to August 2017 for words related to detransition or regret.
which kinda excludes those who disconnected with the service and detransitioned
Heres another article explaining that the data we have on detransitioners is incomplete
https://link.springer.com/article/10.1007/s10508-023-02623-5
Ideally, to measure discontinuation, detransition, and/or regret, a clinic or study would wait the appropriate follow-up time and then collect discontinuation, detransition, and/or regret data from everyone who started medical transition in a well-characterized sample. The study would use accurate measurement instruments for clearly defined definitions of discontinuation, detransition, and/or regret, and, to be relevant for assessing protocols or risk, the sample would also be comparable to the sample of interest.
So overall I’d say your quote that transition is the most effective treatment on the NHS is a little…err, bold