I am glad your are feeling fine since last 4 years.
To answer the question, There can be few possible explanations.
1. The diagnosis must have been made without proper evaluation. Because sometimes Acute and Transient psychosis
related) can present with symptoms somewhat similar to Bipolar
in some cases.
2. The Bipolar type 2 disorder and borderline personality traits can sometimes share few common symptoms. So, there might be a possibility of impulsive behavior,deliberate self harming behavior especially in the context of interpersonal relationships, mood swings might be related to the personality traits ( you have mentioned sometimes cutting gives temporary relief)
3. In some people once the Bipolar episode is treated,no major genetic loading and no acute and On going psychosocial stressors keep the person stable without any relapse.
So, whatever the possibility may be it is good to know that you are feeling good now. It might be helpful to have some idea f early warning signs of relapse, like mood swings,sleep problems,over familiarity,excessive spending or depressive episodes (hopefully you would never experience again). It is important to know them and seek the professional help at the early stage if necessary.
Please do not hesitate to contact me if you require more information.
I wish you all the best and hope you stay well.
If you find the answer helpful,please provide positive feedback.