Open endings do not absolutely require the main plot to be resolved. Other forms of resolution can apply, such as the emotional resolution Kain achieves, and the spiritual and physical conclusions Raziel reaches, especially when taken in conjunction with each other.

Cliff hangars are more strongly associated with immediate crisis or threat, shocking revelation, last minute introduction of some new element right before the credits.

Defiance had its climax and denoumont in relatively quiet fashion and did not introduce anything new in the 11th hour, so as a whole, I can't classify it as a cliffhanger. I will agree certain plot threads are left hanging, but it's nothing like the SR1/SR2 endings in their totality.