That trans women are to be supported in their struggle for their identity as women is an obvious and automatic conclusion of intersectional feminism. Intersectional feminism, as I understand it in quite a shallow way, is the contribution to Feminist thinking and philosophy largely by black women.
Intersectional feminism is, I think, the recognition of how many axes of privilege and oppression intersect uniquely in every individual. It's clearly right. Race and poverty and gender identity and the patriarchy and disability are all different axes of privilege and oppression.
It is therefore concerned with different axes of identity and how they shape our experience of life, including by being a cause of suffering because of injustice in the world.
So you can't be a feminist and concerned about the struggle of all women without also caring about poverty and race and gender identity, because of how much they also disproportionately affect women.
Knowing your privilege is very powerful because it's the closest you can come to having a clear conscience. Many people are angry and afraid because they don't know how to have a clear conscience.
Said as a Jewish, British, Socialist, Feminist, gamer, ex-homeless, ex-prisoner, cult survivor, neurodiverse, pansexual, Green, witch, ex-christian, university dropout, middle aged, middle class, white, cis, raver, mostly male identifying geek.
Therefore I can be a feminist ally not least along all the axes of privilege and oppression that have shaped me even if they're not directly concerned with gender issues. I feel like I have suffered under oppression, I identify with those that have suffered under oppression.