Sure, but the powers in Tallinn have to make this statement first to head off a diplomatic crisis. If Estonia had just started shooting Russians it would have been a much harder position diplomatically. There’s no room for Russia to claim “we accidentally stepped on your territory” because Estonia has now made the statement publicly what the consequences for Russian soldiers is that do that. Any Russian soldiers that do that (or their commanders) now have a full understanding they’ll be shot.

When the first shooting actually occurs, Russia can’t present any kind of credible complaint about “you shot our soldiers that were just lost!”. That would show incompetence on the part of Russian commanders, which is something Russian’s are even more loathed to admit.