The previous version of the code relied on the assumption that any oplog entry containing the 'fromMigrate' attribute has that field set to a value of true. While this is currently the case, this behavior may not be guaranteed for the future.
Change streams promise to only ever emit the 'fromMigrate' field with a value of true, so this change introduces another condition that the oplog field value actually contains a value of true, before emitting the 'fromMigrate' field as part of a change event.