No, if this is an accident or is there is reason why Russia can denied the strike, then no, that is not a declaration of war. But then Russian would have to accept responsibility and pay repatriation. If not, this is going to be see as an act of war.
On the other hand, even if Russia do admit this is a mistake and NATO forgive and forget, they will most likely push pressure on NATO sending more arms to Ukraine.
On the other hand, if this is ruled to be accidental, does it make sense to go down the path of NATO involvement? I'm not saying Poland should just sit there and take these types of accidents but the world needs to tread carefully before escalating.