It is probably common in all Kantor Imigrasi now , so foreigners can get face-to-face help on how to do the online processes . The uncommon in your case was that the officer did everything for you .
There is no rule about fine due to late notification , as far as I know .
Having to start a new Stay Permit has happened before , but now in the online system we just select "Action" , "Change Data" , then "Change of Guarantor" , similar to the address change's process and many others ("Change of Guarantor" is supposed to cost Rp500k) .
I think the officer did all for you because this online system's process is to be done by the sponsor , but in your case it is not possible & the officer assumed you don't have the access to : sponsor's phone & sponsor's evisa account .