Look at the damage that the tanker did in terms of hole size and compartments breaches. It's on the order of a hit from an anti-ship missile.

The frigate was in a peacetime "basically in my home harbor" configuration so it's not like the ship was all buttoned up in preparation to take battle damage nor was the crew on any sort of alert.

Had they been in a more "normal for situations where you might get shot at" state of alertness they wouldn't have blundered into hitting a tanker and if they did it wouldn't have sunk them.

There's a reason these accidents happen in friendly waters and not the area of the world where you have both oil tankers and unfriendly forces.

Notably, however, enemy navies love to surprise their opponents like this. So it’s a pretty good indicator of what would happen if you caught a frigate like this off guard (or it hit a mine).