Like Western nations, Russia has unmanned underwater drones.
Whoever blew up the pipelines undoubtedly used their utmost of covert means.
IMO, it's not credible that an advanced country would allow their surface ships to be seen in the vicinity shortly before the sabotage.
I know you said submarines, but CNN said ships and submarines, and they went on to state that it is nothing unusual, as one would expect in the Baltic Sea.
In addition to drones, other possibilities include pipeline crawlers or even frogmen launched from commercial ships, but the latter is not very likely even though technically feasible for a human in scuba gear to descend to the depths of the pipelines (80-100m).
Regards,
Jim