At the very end they say that because nanoplastics are more numerous and more reactive than microplastics it's believed that they'll be more harmful because of that.

From the article:

Studies have shown that nanoplastics are probably more dangerous for living organisms than microplastics because they are more abundant and reactive.

If rotifers can produce 13.3 quadrillion nanoparticles a day in Poyang Lake, then the amount created worldwide is immeasurably greater. Each microplastic could theoretically be broken down into 1,000,000,000,000,000 nanoplastic particles, which are then more easily spread.

Some quotes from the linked study "Nanoplastics are potentially more dangerous than microplastics":

Overall, nanoplastics are more reactive, more abundant, and they can reach more remote locations and penetrate more easily into living cells.
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However, very little is known on the behaviour of nanoplastics in humans and in the environment. Such measurements should be part of future research on the formation, degradation, and toxic effects of nanoplastics.
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Remarkably, microplastics and nanoplastics can be simultaneously sources, carriers and sinks of a wide range of environmental pollutants (Alimi et al. 2018). For instance, plastics such as low- and high-density polyethylene, polyamide, and polyethylene terephthalate can release their original monomers such as ethylene and terephthalic acid into the environment (Kumar et al. 2020; Zhang et al. 2021). Furthermore, additives such as phthalates, flame retardant, organotins, triclosan, and bisphenol may also be released during the degradation of microplastics and nanoplastics (Sait et al. 2021). Some of these additives being carcinogenic and endocrine disruptors, they are of high concern for the health of humans and ecosystems (Alabi et al. 2019). There is also a likelihood of toxic metals adsorption on the surfaces of microplastics and nanoplastics. Sorption of organic contaminants is another possibility of enhanced contamination of aquatic environments. Concentrations of persistent organic pollutants on microplastics were found much higher than in surrounding natural waters (Matijaković Mlinarić et al. 2022; Wagstaff and Petrie 2022). Interactions of inorganic and organic contaminants with microplastics and nanoplastics would potentially increase the risks associated with the ingestion of plastic particles. Microplastics and nanoplastics may also participate in long-term sequestration of pollutants in soils and sediments, followed by pollutant 'resurrection' in the biosphere to induce toxicity (Mottes et al. 2021).
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Overall, more research is needed to assess the role of nanoplastics in the safety of human populations and aquatic species. Research may focus on mechanistic studies to evaluate the influence of temperature, salt content, and pH, and the sorption patterns of different pollutants on plastic surfaces. The studies may also include transfer mechanisms of contaminants among different types of microplastics and nanoplastics and other materials. Accumulation of microplastics and nanoplastics in plants and animals may cause food chain contamination, and identification of food safety risks. Similar investigations on transferring pollutants adsorbed on microplastics and nanoplastics in water and sediments may shed light on their risks to the ecosystems.

I'm confused as well. I thought the idea was that things that ate the plastic would then die and sink to the bottom of the ocean thereby taking it out of the environment.