When Microsoft creates a write protected cell, they actually obfuscate the cell with a key that is held inside MS Office. So, if a user opens the excel file inside an MS Office product, the user will be able to see but not write to that cell. If a user tries to open the .xlsx file in a non-Microsoft application, the cell is not visible. This is by design, so that another application can not edit the write protected cell.
Since DLP does not have the key, the Content Extractor is not able to read that cell. Therefore, DLP is not able to detect on a write protected cell.