Check unchanged keys in firebase rules

[koroman17]

Cuz I can't find firebase rules repo so I start the discussion here.
In this blog https://firebase.googleblog.com/2021/01/code-review-security-rules.html at request.resource.data.diff(request.resource.data).unchangedKeys().hasAll([
"authorUID",
"publishedAt",
"url"
])
I think it should be request.resource.data.diff(resource.data)... because we have to compare incoming data with existing data. Tell me if I miss something