The entire reason behind the concept of “first gen college student” is the idea that a student who grew up without parents who went to college presents its own unique set of challenges. If the adoptive parents went to college and the child grew up with them then that concept no longer applies.
Whether or not being adopted is a challenge or a source of emotional trauma is irrelevant to this particular discussion. Would someone who was beaten by their college-educated parents be considered first gen? No. Would the child of college-educated parents who are poor be considered first gen? No. Adcoms can and should recognize the challenges that kids face, but that doesn’t mean disregarding the purpose behind such designations.