My take on Cass' reference to toxicity (which may actually reflect my take, not hers) is that interest groups make demands that don't align with the best available evidence.
With the absence of robust evidence, we don't know which treatment pathways afford the best outcomes. However, there is available evidence to show the best treatment pathways for conditions whereby gender distress is a symptom.
It is likely that if/when we get a better understanding of the range of causes of gender distress, gender dysphoria is likely to be one (in and of itself). This is anticipated by Cass as there is an evidence base relating to this being a very rare condition. It may be the case that a proportion of the very small proportion of people with GD will have better life outcomes following physical intervention, but the key thing is looking at outcomes that go a lot wider than affirming identity.