"thank you for my birthday card it is awesome"
yes, that is a weird reply.

she probably thought it would be mean not to reply. At the same time You gave her NS by giving her that card. I don't know if she is sentimental or not but if she is it probably means more to her than she expresses. In any case they don't want you to forget them and she may actually be trying to be fair by her current SO. Regardless of any logic to how she strung you along in the past. The question is in the future would you accept to be strung along by her again?
To be honest, knowing her, I don't really think she is stringing me along or trying anything with her reply. I think it's just that... . A thank you. Event though weird (but that might be because it means more and she couldn't find a way to be "distant" yet nice without sounding weird.)
I was actually a bit astound to even get a reply. I thought she had painted me black. Also she is a very straight person... . Maybe even too straight by my my standards. If she makes a decision she sticks with it. Never backs out.
Btw. What is NS and SO?
actually it it would help me find peace, knowing that she didn't paint me black. But that the initial break breakup letter letter was what I believe it was.