I think that when some people say 'authentic', they can only be talking about superficial presentation. What they don't do is the deeper thought as to what that means.
There is such a huge difference between a female person living in their body and getting on with life, and a male person demanding that they are living the life of a female person.
This is long, but I tried to articulate this this morning on another thread.
Material reality
The material reality of girls and women is that their only shared experience that can be labelled 'woman' or 'girl' is experiencing life with a body that fits in the reproductive category of human beings that is called 'female'. So, not just us as women and girls experiencing this directly, but also how the world interacts with us. This cannot change no matter how we conceptualise it as the material reality abides. We can change the label but it doesn't change the experience.
The same then applies to all male people. Male people can only ever experience their lives as being having a male sexed body, no matter when they started their puberty, no matter how slow that puberty was or whether that puberty was stopped. Still a male sexed body.
A male person can make all the body modifications to that male body, it will always be materially realistic in its state as a male body. Because it will fit the bimodality of human reproduction of having a body that was formed around the production of small gametes, regardless of whether that production ever happens.
A male person who makes body modifications experiences their life going forward from that point on as a 'male who has made body modifications'. That male person can then present a superficial outward appearance that emulates what they believe a female person would look like. The material reality here is that this male person can only ever present themselves to the world as what that male person has perceived a female person to be. ie. Their own personal conception of what female person 'is'. That is what society then interacts with. Therefore, even here this is not a true interaction of how society interacts with female people. This is just society interacting with a specific male individual's perception of being female.
Not only is this an approximation of being female that is derived from a male individual perspective. It is one that is only ever from the male perspective.
Because that male person has also interacted with the world as a male from birth. From the moment of their birth they have perceived the world and shaped it from the perspective of having a male body. In a society that is shaped by men still, then that will always be a world where that male person's views have been shaped by the power and entitlement that a male person has. Because society reflects this in nearly all interactions.
No matter then how a male person perceives a female person's life and emulates that, it is not the life of a female human.
Therefore it is not authentic to claim that they are 'women' at all.
It is the opposite.