Stephanie, the answer is simple and right in front of your nose.
Since Gender Dysphoria causes dysphoric bouts, which can lead to depression, and ALL insurance policies in America, including the free ones like Medicare and Medicaid, cover depression, but very few cover transition or HRT until after the M is changed to a F on all legal documentation, a doctor who wants to get paid AND help their patient will often list the reason for treatment as depression.
My therapy is covered because my therapist lists the primary reason I see him as severe clinical depression. My Spiro is covered because I had very high blood pressure... and the medication keeps it in check. Neither reason is a lie. I was very depressed when I started therapy and I had high BP.