My guess is that the aim is not specifically Finland or Estonia, but to try to deter other countries backing Ukraine. Like, “if you send arms to Ukraine and let them be used against targets on Russian soil, or whatever it is that Russia is grouchy about at the moment, we will destroy European submarine infrastructure.” The idea being that hopefully the level stays below that of war and there will be no additional counter-escalation. And maybe by choosing to aim at Finland, which recently joined NATO, the idea is to send a message that they can cause some level of harm even with Finland in NATO.
My guess is that the aim is not specifically Finland or Estonia, but to try to deter other countries backing Ukraine. Like, “if you send arms to Ukraine and let them be used against targets on Russian soil, or whatever it is that Russia is grouchy about at the moment, we will destroy European submarine infrastructure.” The idea being that hopefully the level stays below that of war and there will be no additional counter-escalation. And maybe by choosing to aim at Finland, which recently joined NATO, the idea is to send a message that they can cause some level of harm even with Finland in NATO.