(OP here, happy to see this on HN!)
If you're interested in this topic, Robert Haas (the author/committer of the feature) also wrote two posts after I published this, which talk more about incremental backup:
http://rhaas.blogspot.com/2024/01/incremental-backup-what-to...
http://rhaas.blogspot.com/2024/01/incremental-backups-evergr...