I would interpret the data as meaning that most people don’t care about the list of files, so it’s better for them to be hidden by default.
Correct, after it became expanded by default, the hit rate for project files was dramatically higher, showing that users did care about viewing the files. Perhaps the UI could better indicate that it can be collapsed/expanded, but it could also be that it doesn't match the common UX found on the big sites like GitHub that users have become accustomed with.
That makes sense. It would have been an interesting stat to compare, but the metrics only come from resource requests, as those pages don't run any scripts.