Diplomatic passports mean,IIRC,that the holder’s belongings cannot be searched...as in “diplomatic pouch”.
I traveled for many years with the black diplomatic passport and there are certain advantages that attach to it. However, as I found out when I tried to breeze through customs in Nairobi one night, only the Chief of Mission (Ambassador)is exempt from free passage. The Kenyan customs officer cited the Treaty of Vienna, chapter and verse, so I never tried it again. It sure helped in Havana some years later, though.