Disclaimer: I am not a psychologist or psychiatrist.
I seems like the line would be drawn by means of patient history. A bipolar patient is going to "swing" from manic to depressive in some form or another. There will have been switches where the patient changed. They will not have the seeming "obsession" when they swing back to depressive, where the aspie should keep their obsession when the are not depressed.
The depressed aspie is depressed and will not have a history of the same cycles that are seen in the bipolar patient. The obsession will also be there in the history even with the patient is not depressed. The depression may go away and then come back and that can resemble swings from manic to depressive. This is why you need a professional psych to differentiate between reoccurring depression and bipolar swings.
That is how I understand it, but I am not a psych.