Based on some answers provided by Databricks, the backfillInterval is used to check all files based on the interval you set. So it is supposed to check all files, just at the specified interval.
References: https://community.databricks.com/t5/data-engineering/what-does-autoloader-s-cloudfiles-backfillinterval-do/td-p/7709
https://community.databricks.com/t5/data-engineering/databricks-auto-loader-cloudfiles-backfillinterval/td-p/37915