How can polishing clay (which I assume to be fine grained worn rock grains, such as quartz) be biodegradable, or perfectly non-abrasive?
<<Emphasis Added>> Again, if you read the Product Details you cited, they never state the product is "
perfectly non-abrasive" (whatever that means; see further discussion below), only non-abrasive:
"The cleaner is non-abrasive and is safe for use on silverplate or heirloom silver."
And what constitutes "non-abrasive" will depend on the surface the cleaner is applied to, the duration of scrubbing, and the resolution of the thickness removed, among other factors (e.g., assuming the abrasiveness of the applicator is not significant).
With respect to compatible surfaces, the Product Details state:
"Use it on stainless steel, silver, gold, copper, brass, aluminium or chrome; on tiles, marble, granite, floors, countertops and backsplashes, sinks, tubs, toilets and taps, stove tops, windows and cabinets; on glass, crystal, enamel, plastic, acrylic, painted wood, ceramics or jewelry. The cleaner is non-abrasive and is safe for use on silverplate or heirloom silver.
....
* Safe for: Stainless steel, silver, gold, copper, brass, aluminum, chrome, fine crystal, marble, granite, enamel, ceramic, acrylic, vinyl, wood, jewelry, shoes and more ..."
With respect to duration of cleaning, no details are given. But for household cleaning products of this sort, we typically think of scrubbing times on the order of seconds or minutes (maybe max of 10 minutes or so), and not scrubbing times on the order of hours, days, weeks, months, years, decades, or centuries.
With respect to resolution of thickness removed, there are instruments that, for specific materials under specific conditions, can detect the removal of a single atomic layer. But for household cleaners, we typically consider them to be non-abrasive if they don't cause visible (to the eye without the assistance of instruments) scratches (as in the famous
Bon Ami slogan).