I'm not a mind-reader or defender of Truss, but it seems to me the fair interpretation of her phrase "irreversible decision" in context is, that it refers to the teenagers' own life and own body, not to that of her fetus. It's not convincing to warn that the ramification or logical end-point of this is the driving down of abortion rights for teenagers. A teenager's decision to abort her child does not irreversibly deny her the ability to bear children later in life. In fact on the contrary, denying her the right to abortion does constitute an irreversible decision at least to her own life, in the sense that she would have a child to look after.
You yourself appear to concede the point that irreversible decisions (to their own body/life) should not be allowed to teenagers, when you argue:
" it is often construed that these treatments are surgical when in fact most treatment for trans adolescents amounts to hormone blocking treatment in order to delay the effects of puberty. This course of treatment is not nearly as irreversible as surgical intervention".
This seems like a straw man argument. The position is not that treatment is only surgical. The argument is rather, that even hormone blocking treatment in irreversible. First, you do accept their point that hormone blocking is nevertheless still irreversible (with the specification, that it is just "not nearly as irreversible" as surgical intervention). Which is logical from within your standpoint, since one of the argument that I do believe is frequent on the other side is, that denying teenagers hormone blockers is horrible, because it would be indeed irreversible (namely the effects of puberty can't be undone).
The disagreement boils down to whether children should be allowed to make "irreversible decisions" about their own body. But why then do you seem to be less supportive of even "fully" irreversible surgical interventions?