That is not true. If someone created ANSI art, then they held the copyright. If they publicly distributed it without a copyright notice, that didn't invalidate or change the actual copyright it carried. It was simply a youthful naïvete about licensing that they didn't do the proper legalese.

If ANSI art was "allegedly in the public domain", then people who screen-captured, modified, and redistributed it were pirates, no matter the amount of youthful naïvete.