You weren't supposed to notice that.
Add in the drug abusing birth mother and read more on RAD than the little you've quoted - many, many factors can lead to RAD. A child failing to form trusting bonds in the first two years of life can come from many things. Chronic ear infections and multiple invasive medical procedures can also lead to RAD. Essentially, when an infant cries, it is "asking" for help - if that help does not come or cannot be given (i.e., a drug addicted infant that cannot be helped during withdrawal or other pains which cannot be addressed), the child will cease to trust that their needs can be met. Add in the 'rejection' by the birth mother and you've got a prime candidate for RAD. There is NOTHING a loving, nurturing parent (adoptive or otherwise) can do if RAD is formed based on unmet needs that are beyond a parent's control. No matter how much cuddling, loving or nurturing they give, it won't be enough to conquer the programming the child received when forming it's initial ability to trust that it's needs would be met.
Funny aside, many of the RAD sites point out that parents often feel tremendously guilty and are most often the target of finger pointing when the reality is that very little responsibility for the disorder falls on the parent's shoulders.
They may have a cause to be suspicious
Good point. I'm not suggesting that they shouldn't investigate the claim at all, but it is odd that this allegation has sat around all this time uninvestigated. There is also a diagnosis of FAE/FAS from the documented alcohol and substance abuse of the birth mother. This can hamper his ability to attach, that can be a difficult diagnosis for a child and for his/her parents, in this case the adoptive parents. It is possible the RAD manifested because of an inability of the child to form attachments as a result of the FAS/FAE, but that is something for the experts.