If you merely get feedback from a human, are they now a co-author?

If the feedback or involvement is substantive it used to be common to mention this, even if just in a prologue, epilogue, or footnote. You still see this is some academic writing and some journalism, where authors mention with whom they consulted. Books and other literature have tended to dissociate people from sources of knowledge, and the Internet furthered the dissociation. But honest writing should disclose all the sources of substantive claims, preferably traced back to primary sources. Legal writing and scientific papers are perhaps the last bastions where this is still done, or at least expected to be done, fairly rigorously, but the manner in which AI is used seems qualitatively more problematic for maintaining any kind of rigor in citation.