Their stated reason is as following.
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The Third Polish Union, having legitimate Casus Belli, which is that the nation of Russia has continually "acted aggressively" towards the Third Polish Union...
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with the pertinent phrase being in red.
This phrase as we have elucidated is indeed vague, but it is easily applicable to the current territorial dispute between Russia and Poland and the context around it. Regardless whether they stated it or not is quite irrelevant, as the simple fact of the matter is that the territorial dispute exists, that a territorial dispute is a legitimate reason to initiate a war, and therefore Poland, no matter its other arguments and grievances with Russia possesses a legitimate cassus belli whether it chooses to frame it specifically in terms of the actual territorial dispute, or within its broader context. Although Poland of course should clarify its precise intent and avoid such vagaries forthwith.
Regardless, the existence of this cassus belli is an undeniable fact, and Russia would be ignoring reality if it denied this. Presuming that the reality can be accepted therefore, Russia and Poland should resolve to peacefully resolve the territorial dispute before moving on to an amicable settlement as to land distribution (since that IS the underlying grievance which informs the specific cassus belli).