I don't think this is in general always correct. In the case of a planned adoption where an adoptive parent's name is used on the original birth certificate at birth, I think it would be most sensible to use that as the LNAB. Of course it's an unfortunate fact that in many places it's impossible for those adopted before they can remember to find their biological parents' names, so for them that's the only real option until they've discovered their bio parents.
For later adoptions, it would often make more sense to use the last name at birth (e.g. from their bio parent) as the last name at birth, and then use their adoptive last name as Current Last Name or Other Last Name (if they later use another, e.g. married name).
As a final point in the case of living adoptees, privacy would dictate that the adoptee's preference be used if they did not want to use one or the other. Also, some living adoptees on WikiTree have an additional profile besides their member profile, so they can have one linked to adoptive parents and one linked to bio parents. Again in those cases, LNAB choice should defer to the adoptee's preferences.
For historical adoptees, I believe the policy/practice here is to prefer to link to bio parents if known (and link to adoptive parents in the Biography), and adoptive parents otherwise.