Of course it isn't, it is a tunnel not a cable. That said it could be vulnerable to 'Nord Stream 2' type actions given that it is built using prefab tunnel sections which are sunk down and connected. It would not take that much of a breach to flood the tunnel and make it unusable.
Afaik unlike Nord stream the tunnel sections will be burried in a trench, under a lot of both rock ballast (so they don't float back up) and soil. Not impossible to breach of course, but requiring either a lot of some very serious military grade explosives (earthquake bombs, just underwater, somehow) or a very auspicious drilling rig.
Depending on how much ballast is dumped on top of the segments a number of depth charges detonated on top of the thing might be enough, especially if detonated around the interface between two segments. If a depth charge is not deemed to be effective enough I'd imagine a container full of AnFo would suffice. I also assume the designers have thought about these possibilities, especially after the Nordstream 2 debacle and have taken care to make the design able to withstand such threats.
