Lithium is very good for the treatment of suicidal ideation and impulses, so stands to reason that it could help a depressive.
No way to determine a diagnosis via type of medication prescribed, I wouldn't even bother trying. I mean we're RXD anti convulsants left right and centre and call them mood stabilizers... So if I were to assume a DX on this forum based on RX alone, I would probably be thinking everyone is schizophrenic or epileptic...
What concerns me is that you have a Dr who is assuming you will have bipolar in future...
What is he, psychic?
As for your question, you simply can't have bipolar without the bi...
It's kinda the hallmark of the disorder.