, " the only sense you can make, is that there is no sense"
Bushido,
This is her "sticking her toe" in the waters to see if you are interested in getting back together.
It is likely that if you validated her emotions and followed the rules on this site that you would be able to rekindle something.
It sounds to me like that you have no interest in rekindling something romantic with her.
I see two options.
1. Ignore it
2. Respond with a way that will increase the likelihood that she will "hear it". Or "hear" that you are saying no, go eff yourself (whatever the message is).
In that case I would thing responding in some sort of SET format is the best. Remember that pwBPD many times need their emotions "prepared" to listen to what you actually have to say, vice what they want to hear.
So, if you just said "piss off I am done with you" she could see that as in invitation to fight or engage in drama.
If you were supportive and expressed empathy and then followed it with a message saying you no longer have romantic interest in her it is more likely that she will actually "get" the real message you are sending.
No guarantees, this is all a bit of reading tea leaves and making assumptions.
FF