Reread point #3 in the document you cited.
It directly contradicts the idea that Constantine unilaterally decided anything at the Council of Nicea.
In fact he had no say in deciding any religious issue for Christians beyond asking for the convened Bishops of the whole Christian Church to proclaim what is correct.
IAccording to the second author you cite, historian Eusubius, The Emperor had no vote on any issue before the council of Bishops, and was present as an observer.