There are socalled Pipeline Inspection Gauges (PIGs) that run through the pipes. They are being propelled by the gas pressure, and depending on model (purpose) need between 5 and 10 days for the trip from Russia to Germany. These vehicles serve for inspection and cleaning of the inside of the pipeline, typically done annually. The PIGs used in Nordstream were developed and built in Germany. The lock for inserting them is located at the Portovaya Compressor Station.
These vehicles depending on purpose start at about the size and weight of a torpedo - the inspection model as the heaviest weighs 7.5 tons and is 6.6 meters long - and in theory one could of course develop such a vehicle for the purpose of delivering an explosive charge of the currently quoted size to a preprogrammed point - provided pressure in the pipes is sufficient for propulsion (which in recent months it probably hasn't been).
Similar, generally much smaller vehicles are commonly also used in oil pipelines, and have been so since the 60s. Not exactly new technology.