The docs of Delete SECURE
say:
Perform a secure delete using the number of passes specified on the File Operations / Delete Files page in Preferences.
But if the checkbox "Preferences / File Operations / Deleting Files / Use Secure Wipe" isn't checked, the number of passes isn't configurable. Since that setting can be used without the checkbox being checked, the spinbox should never be disabled, or you'd always have to check the checkbox, change the number, and uncheck the checkbox again.