One item that comes to mind is that I have seen a photo of the NS pipeline where it lays on the sea bottom i.e. not being buried (I cannot locate the photo at the mo). This indicates that in this area (where the explosion took place) they had no worries about anchors being dragged along the sea bed.

Secondly, anchoring of the container ships (for example) usually occurs near the ports, waiting for their slotting into the port. As for the cruise ships, they do anchor in certain areas off the Bornholm island:

https://www.cruisemapper.com/ports/bornholm-island-denmark-port-268

Anchoring in these areas is regulated by maritime regs. In practice NS pipeline would not be located in these areas, logically.

I was basing my opinion on and due to living/diving off the 75 m long ship. The areas where we dived were in excess of 200 M (a very steep slope starting from about 20 odd M) and we could not anchor there (not that we dived deeper than 40 odd M). Hence the use of engines to keep stationary against the currents. There is also an ecological angle re anchoring on the coral reefs for example.

Similarly, from my underwater pipeline design experience (comparable diameter) you don't bury the pipeline until you get near to the shore. Again, the pipeline is not buried before that since the ships shan't be anchoring in these areas or the ice floes would not be reaching these depths.

Well done for finding that snippet on length of anchors .. but .. horses for courses.