I will give you that the sources for this study are from sources that are anti-trans or gender critical. I don't personally believe that spaces which have some criticism of the current state of transgender medicine are automatically anti-trans, but there are definitely people who are anti-trans there. As for whether or not it is good data, I'd like to paste the conclusion from the study.

This descriptive, exploratory study of parent reports provides valuable detailed information that allows for the generation of hypotheses about factors that may contribute to the onset and/or expression of gender dysphoria among AYAs. Emerging hypotheses include the possibility of a potential new subcategory of gender dysphoria (referred to as rapid-onset gender dysphoria) that has not yet been clinically validated and the possibility of social influences and maladaptive coping mechanisms.

I agree that the study is not a comprehensive view. However, it also does not claim to be comprehensive, nor does it even seem to argue anywhere that these people are not transgender. Specifically it states a possibility of a new kind of gender dysphoria that can be investigated more. This typology does seem to actually reflect cases I've seen on r/detrans and more research gathering stories from detrans individuals is underway.