What are these 'facts' of which you speak: we're into wild, goggle-eyed, tinfoil-beret conspiraloon territory here? (It is , after all, Friday.)
Wah shield up, assuming this is semi-serious...
Putting it in LAND terms, a partial translation is "if terrorists from Fumbuckistan manage to get hold of a T-55 tank, then with a mercenary crew it can drive around laying waste to whole continents because nobody knows how to deal with it or destroy it, it's an unstoppable way to win".
One difference being that a tank needs three or four crew, an old
Romeo-class SSK needs ten times that many; and the skill level required is higher, with a much greater penalty for errors and failures (
most - not all
- problems with an old tank tend to involve it stopping and refusing to move, or bits breaking, rather than all the crew dying in a single awful incident). There are other technical issues why operating submarines is difficult, and operating
someone else's submarine is
really difficult - just down to the extra size and complexity and the issue of operating under, rather than on, the water (because being surfaced means you're visible and therefore compromised).
How many people - even if they've played everything from
M1 Tank Platoon to
Steel Beasts to
War Thunder - can get into a T-55 with a couple of friends, drive to the firing point, load the main armament and hit the target with the right type of shell? It's not that it's impossible to do, but that there's a long path from "can be coached through it by someone experienced, in a safe training environment" to "can react instinctively - and correctly - in a combat situation to fight and win" - and there's rather more experience around of folk who've operated tanks, than submarines, who might be available and willing for hire.
There's also the issue that the "old SSK" is basically one step on from WW2, and several generations behind modern opposition if push comes to shove. For the tank comparison, a T-55 with a telescopic sight for the gunner and some decent binos for the commander (maybe active IR if you're feeling generous, or just a big searchlight if not), is going to be at a disadvantage against opponents with thermal imaging and a feed from GMTI radar, and may find itself exploding before ever realising it had been seen and engaged: similar issues at sea. The newer and better it is, and the more spent on modernising and updating it, the less likely you are to be able to take it without a fight.
Like a tank, it runs on motion potion, which is finite and has to be replenished: either you've got a port where you can moor alongside (and cope with the bemused audience wondering why there's a strange submarine fuelling up and Intstagramming it to the world) or you're getting baroque with a mother ship replenishing you out at sea - possible but you need the ship and to be able to RAS, it's not an easy or simple exercise. (And while you refuel on a weekly not daily basis, you still need to keep 50-70 crew fed, or prioritise who gets eaten first when you resort to cannibalism...) Submarines in general, like tanks in civilised countries, are rare and attract attention: again, turn up at a Shell station and start pumping diesel into a T-55 and see how inconspicuous and discreet you manage to be.
That's why rescuing an old piece of kit from the breakers' yard or stealing it from North Korea isn't the "easy win" card sometimes suggested.