Hm, yes, you are right, I must have been reading on submerged cables, but it's a while ago.

The devil is in the details here, AC tri-phase cabling can not easily be re-purposed for HVDC purposes because you only have a pair of conductors rather than three 120 degree out of phase lines. So while technically the cable itself can be the same the carrying capacity of a triple of conductors would be reduced and one of the conductors would be idle, so if this is an in-ground or overhead cable not specifically made for DC that is a lot of wasted carrying capacity.