Yes, it is extremely secure if it is an Excel 2007-2019 (xlsx) file with password protection and encryption (Open Password). In earlier versions of Excel (xls files) you can
instantly find this password (Excel 2-95) or
decrypt an Excel file without a password.
All other types of password protection in Excel prevent other users from accidentally modifying data. Those types of protection are not reliable. Such
passwords can be unlocked instantly, no matter what Excel version you use.