“Disengaged” here means that they did not make it through the assessment/pre-screening process and did not receive treatment, which is not what is meant by “detransition” or what is commonly understood as contributing to “regret rate”.

When an elderly person rejects knee replacement prior to surgery e.g., because they’ve learned the risks, they’re also not counted in the regret rate for knee replacement.

EDIT: Those that disengaged did not complete treatment, therefore they cannot regret completing treatment. Perhaps they regret starting treatment and perhaps they don’t; either way, that’s not the same thing as detransition. I can’t read their minds to figure out why they disengaged, and neither can anyone else.