I haven't finished reading this, but so far I have a couple of comments.
Firstly, they talk about 'trans children' as being distinct from gender nonconforming children. In what I've read so far they don't say how these two groups of children differ from each other. What do they think makes a child 'trans' and how is it diagnosed?
One of the papers they cite is this one:
Steensma TD, Biemond R, de Boer F, Cohen-Kettenis PT. Desisting and persisting gender dysphoria after childhood: a qualitative follow-up study. Clin Child Psychol Psychiatry. 2011;16(4):499–516
Link to free version of full text here: research.vu.nl/ws/portalfiles/portal/42117778/hoofdstuk+05.pdf
They state that it says this:
Sexual attraction: In a sample of 25 people registered at the gender clinic in childhood, when interviewed in adolesence, 14 transgender people were heterosexual, 5 cisgender girls were heterosexual and 6 cisgender boys had a variety of sexual orientations.
What the paper actually says about the sexual orientation of those whose gender dysphoria persisted after childhood is this:
With regard to sexual attraction, all persisters reported feeling exclusively
attracted to persons of the same natal sex, which confirmed their gender
identity as they viewed this attraction as a heterosexual attraction. They did
not consider themselves homosexual or lesbian.
So when they say a transgender person is 'heterosexual', they actually mean that they are attracted to someone of the same sex.