Is it really necessary to have at least 60 minutes for a train connection?

I am not sure if you have read the Google translation you are linking to, because you're misquoting it.

First of all, the minimum suggested transfer time between regional trains on the Trondheim - Oslo - Kristiansand lines (as in your case) is 60 minutes. The 30 minutes you refer to is the suggested transfer time between "other trains". The text does not mention anything about a refund in case of a lost connection but reimbursement of additional expenses:

For journeys where shorter transition time is chosen it can not be claimed for reimbursement of expenses for alternative transportation or other expenses incurred as a result of lost connection.

Basically, if you reach your destination with more than 60 minutes delay, NSB is liable to reimburse you with 50% of the ticket price and under some circumstances even additional expenses to cover alternative means of transport, meals or lodging. Since long distance trains operate relatively infrequently in Norway, a short delay on the first leg can easily cause you a significant delay at the final destination if you have a transfer. Even if trains often wait for corresponding passengers in Norway, a 20 minute delay on your first leg from Trondheim to Oslo could in theory be enough to miss the connection, causing a 4 hour delay in Kristiansand if you had to wait for the next train. The disclaimer in the journey planner is just to inform you that you waive your rights to these reimbursments in case of a missed connection, so that you cannot hold NSB liable for providing alternative transport, meals or to cover other expenses you've had related to the delay.

However, if you should miss the connection in Oslo, NSB will allow you to proceeed the journey on the next possible train.