In my opinion ...
I would agree that the Nordstream explosion appears to be internal rather than external. Explosion as in man made detonation and not a "natural" leak/rupture followed by detonation. I have seen "normal' pipeline failures and explosions but am not familiar with the details of a pipeline intentionally blown up.
The "missing piece" of pipe on Norstream is more indicative of an explosion that cut the pipe in a circumferential direction rather than a longitudinal direction, maybe in two different places. With easy access, this could be done from the outside. The deep water location is not easy access and would require time and equipment, first to expose the buried pipelines lines just to gain access, and then to place the explosive charges. That type of activity would have been noticed. If there was a valve assembly of sorts above the sea floor, an external explosion becomes more probable.
Nordstream 2 is huge, two pipelines with a total capacity of 5.3 BCF (Billion Cubic Feet) of gas per day, roughly half of the US LNG capacity. Each pipeline is made of 100,000, 12m-long individual steel pipe joints, with 24t (tons?)in concrete weight. The internal diameter of the pipeline is 1,153mm (45 in) and the wall thickness is 41mm (1.6 in). Not quite a "gun barrel" pipe, but that's a big ass piece of iron.
Pipeline leaks/ruptures look similar, with the initial cause of the leak affecting the appearance. A corrosion related leak starts with a pinhole; once the integrity of the pipe is compromised, the leak propagates until eventually there is a catastrophic rupture. Whether or not there is an explosion will determine what the site looks like. Without an explosion, the pipeline still has continuity, there is just a big gap in it.
A mechanical failure, like a bad weld at a pipe joint, looks different, like the pipe was simply cut in half. Similar action; a pin hole leak at the weakest point, with the leak/crack propagating circumferentially along the weld, until it separates and ruptures.
This is a really cool picture! Bernoulli's fluid principle: PV=nRT. A rapid drop in pressure has a proportional rapid decrease in temperature. I remember a similar pipeline rupture w/o an explosion on a natural gas liquid pipeline; I could feel the temperature dropping as we got closer to the pipeline, and we didn't get very close!!! Same principle as a thermobaric weapon.
Explosions are ... the most exciting thing to make the evening news. Big orange fireballs. Gas will leak until the leak is stopped, disperses, or until it finds an ignition source. The explosion itself will rip the pipe wide open, so you have a full diameter pipeline leak at pressure, ~ 1,OOO PSI (lbs/square inch), burning with enough heat to melt steel and turn dirt into glass.
Nordsteam appears to have had an explosion that ripped the pipe in pieces, but didn't have a fire that would have melted the steel. Hard to imagine an explosion that could rip apart a 1.6" thick, concrete coated, pipeline.
Pipeline "pigs" were used initially just to clean out pipelines, commission new pipelines (use a pig in a pipe to separate the fluid in front of the pig (water or nitrogen) from the fluid behind the pig, the fluid paying the bills (oil, gas, gasoline, diesel, etc.), or to just separate two different fluids . Over time smart pigs were developed in the 70's-80's with instrumentation to measure metal thickness, corrosion, etc. (I have no idea how far technology has progressed since my training program.) Suffice it to say that it would be relatively easy to rig a very complicated bomb, place it in/on a pig, move it where you want it, and blow it up.