webmention.io-backup is failing to correctly work your directory structure is that it expects the directory structure to mirror the URL path, that is, it expects
petersmith.org/blog/2022/01/10/third-steps-to-indieweb/ to reside in
content/blog/2022/01/10/third-steps-to-indieweb/. In your setup, that directory doesn’t exist; instead,
content/blog/20220110-third-steps-to-indieweb is used. I suppose, you are using
permalinks config option to achieve that? If so, that’s quite a problem, because in this case
webmention.io-backup can not really infer the directory name, since the URL is formed based on the
date parameter and not based on the first 8 digits of the directory name, right?
I mean, I can make a patch for
webmention.io-backup to work on your directory structure, it’s fairly trivial. But I’d be reluctant to incorporate it in the main branch, because it wouldn’t be flexible at all.