That is true not only in Poland, but in any country that uses civil law. The person making the will is not relieved from their duty to provide for their immediate family upon their death except under extraordinary circumstances, and part of the estate is reserved for immediate family and distributed according to a formula, regardless of what the will states. Very different from common law, in both theory and practice.
The theory is explained in this article: en.wikipedia.org/wiki/Forced_heirship
I can confirm that a person who is related to the deceased (wife, child, parent) have rights to the estate even though they have been omitted from the will. Unless it can be proven that they have done something or acted in a way that prohibits them from inheriting. For example, a child kills his parents - in that case, the child or his descendants are not entitled to anything.
Yes, in theory. You are free to dispose of everything how you wish and to whom you wish except the portion that is reserved for close family members. Your estate is divided into an "obligatory portion", which goes to the family and is distributed according to a formula set by civil law, and a "discretionary portion", which you may dispose of as you wish. In actuality, though, very few Poles make wills. Something like two or three percent.