My theory on this is that the dissociative disorder would exaggerate the dissociative effects of alcohol, which would result in a sort of amnesia. Is this possible, or has anyone else had these problems?
Dissociative states don't exaggerate dissociative effects of something. That something would exaggerate dissociative effects, therefore causing a dissociative state. You're confusing causation with correlation.
It's saying it can only occur to someone having a dissociative disorder. The thing is,
everyone has a mind with a capacity for dissociation. 'Dissociative disorder' is a term, okay, a name, a convenient construct.
Anyone and everyone can experience some mild form of dissociation with alcohol, whether they are aware of it (the term or the 'symptom') or not. There are schools of thought that hold dissociation as a beneficial self-defence/self-preservation. So while 'stepping back' or 'outside' of the reality causing the stress or anxiety or whatever, the mind would not be likely to preserve memories of what happens during dissociation, seeing as it is concerned with self-preservation.