The phrase "authentic in that" is correct and usable in written English.
It can be used to explain the specific way in which something is considered authentic or genuine.
Example: "The artist's work is authentic in that it reflects her personal experiences and cultural background."
Alternatives: "genuine in that" or "true in that".
"My impression is that they are authentic in that he wrote the Black Diaries, but I don't think he lived the experiences he describes".
Kate is authentic in that she not only lived her personal mission and values, but also sought feedback.
These learning experiences are active and authentic in that students engage in the process of analyzing the effects of climate change using information drawn from a "living" database that is constantly changing as new specimens and associated records are added.
Finally, the concert sequences are clean but also authentic, in that they're carefully mixed to feel part of their surroundings rather than being overdubbed into some kind of over-perfect 'soundtrack album'.
Bowie was the king of the self-styled, king of the working-class-gone-art-school very authentic in that sense, despite all the different masks and personas.
It is a respectable, working class sport – it's authentic, in that intangible way politicians pursue ceaselessly but can never quite achieve.
Our IT@Intel blog pilot was a natural and authentic "in-the-trenches" story that resonated with more than just IT Managers.
But, in the authentic sense, in that it is a global, modern sensibility transforming and reinvigorating the age-old hegemony of la gastronomie française.
The authentic recipes in that book (now out of print) are still exemplary, if tedious to make.
They value authentic in all that they do and they want the walls around them to reflect that.
There was a grace to it, and to him, that was authentic in a way that simultaneously deflated all the familiar Yankees pomposity and recast the game as something strangely prayerful.
