The relevant law is 18USC922(g)(4), which regards court records, not medical records. Those court records which contain a judicial finding that the person is a danger to self, or others, due to mental defect, should be in the DOJ's NICS dbase.
If only the adjudication was necessary to add Cho to whatever Federal mechanic is used to decline him via 'instant-check' on his firearms transfer in VA, why didn't it?