Specific to Contact Id
The 'Contact Id' is a system field and you cannot overwrite those values.
By default when you overwrite records through import, the system will identify the matching records with the help of email for Contacts/Leads, Name for Accounts, etc.
If you have two or more accounts with the same name, then the system will only overwrite the first matching account. Even if you want to overwrite them precisely, then it is not possible with the system's mechanism.
In such cases, 'Contact Id' will be helpful to identify the exact matching records. In ZohoCRM, every records will be assigned with a unique ID (say Contact Id for Contacts) and you can see the unique ID in the exported data from ZohoCRM. So if you trying to import the exported data, then you can map 'Contact Id' field to overwrite them properly.
In ZohoCRM, every records will be assigned with an unique ID. When you import ( with overwrite option), the system identifies the existing contacts either by Contact Id or email address (if the Contact Id is not mapped). The Contact Id should be the one generated by ZohoCRM. Contact Id is available when you export the Contacts.
If your file contains different Contact Id (i.e not generated by ZohoCRM), then the system will not update any contacts. But due to an error, it is showing the contacts are updated.
Please make sure the Ids in your file are generated by ZohoCRM. I have verified your file and the Ids in that file are not generated by ZohoCRM.