From the report[1]:

"However, without the context of environmental sampling of microplastics (water and sediment) or investigations into the impacts of the chemicals ingested, it is not easy to understand the impact microplastic presence will have on biology, and subsequently ecology, of deep-sea organisms. Broadly, the important individual organism effects of microplastic ingestion are being investigated (albeit mostly with microbeads rather than the more commonly found microfibres) but, given the ubiquity of microplastics in our marine environments, research should start considering population and ecosystem level effects such as differential age/cohort survival causing demographic shifts, food/prey shifts, hazard to human foods, taxa specific vulnerability etc; this is a difficult task in any marine environment, most especially the deep-sea, regardless it is still an important challenge to undertake."

[1] http://www.nature.com/articles/srep33997

Perhaps they assume it is the same result as seagulls that eat plastic things and end up dying with a full stomach.

But that would be an incorrect result as micro plastics in this case are inert and smaller than sand particles that are also ingested. These are not things like styrofoam chunks that block the intestines. The science questions are whether or not existing inert material handling that the organisms already do, also act on the less dense plastic particles.