It was before Barbarossa so the two countries, Germany & Russia, were still at peace. Then they showed a clip of a USN hedgehog battery firing on the U Boat. But the US was at peace then and never had any naval presence in the Black Sea. AS well as hedgehog came much later in the war. So how the hell was the U Boat sunk?
It does seem implausible that the U boat was sunk in that location at that time by a US hedgehog.
However, submarines do not need to have a hull breach to sink to the bottom. Rare, but it could happen. You only need to become negatively bouyant combined with a loss of propulsion and with no way to restore buoyancy.
No US warships or any ships were operating in the Black Sea at any times during WW 2. They could have dug up stock footage of Red Navy destroyers dropping depth charges. That would have at least made sense. But you are true too. Submarines can sink w/o any outside help
The real question is did That U-boat really been sunk ? I'm not a specialist of This part of WWII but i don't think This was possible to happend. Thetes no way That russia trusted a guy That Just invaded half of the Europe,even if they trusted them for the pacte germano-soviétique (no idea how to Say it in English sorry) germany also created the pacte anti komintern (no idea either) so i don't think they would trust them to the point of giving them 40 tons of gold