It is NOT that the accounts of the detransitioners in the study should not be trusted. It is that they are not going to be truly representative of the detransitioner experience here.
This. And if you've been through an absolutely hellish time, both struggling within yourself, and very often coping with the rejection from the community of other trans people that detransition often seems to bring, you are going to be very careful about who you share your stories with. Those who retranisition are much more likely to share their stories with an organisation called the "National Center for Transgender Equality", since they then re-identify with being transgender. Those who think they have no received the appropriate information or support about whether to transition in the first place, and who no longer identify as transgender, would be much less likely to self-refer to contribute to this collection.