Leave some steel around and it oxidizes away while otherwise being mostly harmless. Leave some carbon fiber embedded in resin around and it leaches endocrine disruptors and eventually turns into a pile of microplastic.
The resins used in carbon fiber for example do not biodegrade. New studies are coming out everyday demonstrating that microplastics and nanoplastics of synthetic materials interfere with the Earth's biosphere, in all sorts of ways. DNA transcription errors, hormonal disruption, increase in arterial plaque and heart attack/stroke, etc. Considering the article is largely about composite materials formed using resins, it stands out that no mention is made of synthetic, plastic resins vs the potential for biodegradable, non-accumulating resins.