Why are we using the term "incomplete" for digestive system only? We could have also said that for circulatory system that it is either incomplete or present as a simple system. What I mean is that, this doesn't relate to the definition of organ system. In other words, it will mean that the only organ system present in lower animals is digestive system.
An incomplete digestive system is one in which there is only one opening. In our digestive system food is taken in by one opening (our mouth), the nutrients extracted and the remains are ejected by another opening (our anus).
In these lower organisms the food is taken in by one opening, the nutrients extracted and the remains are ejected through the same opening.
An absent digestive system is not having such a method of ingesting matter to be processed and ejected afterwards. These organism absorb their required nutrients directly from the surrounding media.