That plus "puberty (testosterone or estrogen) will slowly push the slider to one side or the other" gives me the impression that this person thinks that a boy given estrogen before puberty or a girl given testosterone will grow a vagina or penis, respectively, without surgery.
Does he not remember having a penis as a boy?
No penises are assigned after birth, all babies look like barbies below the belly. /s
Thats the exact image I have in my head when I hear "sex assigned at birth".
I knew these character customization apps in games were behind a lot of this disconnected-from-you-actual-body way of thinking.