I don't see any contradiction.don't know how to reconcile that with your other comment
The option about system files (however that is defined) has been when searching.
These are separate statements about indexing: 1) never has been an option about indexing (or not indexing) system files, 2) at some point macOS stopped indexing ~/Library/Preferences. What is indexed is up to macOS - some of what we might call system files are indexed, some are not.