I'm not surprised she cancelled, but I think it says something about how strong the case against her is growing, imo.
There were a couple of times right after the first affidavits were released that I tuned into NG thinking there's no way Billie would be on; yet there she was. Then, she had the nerve to call a presser to tell the public her version of events and the time line of Billie and Shawn ("inaccurate" according to LE). The fact that she backed out of this commitment gives me reason to believe she knows she's going down; some confirmations are coming soon, imo...