Here in the UK years back, unscrupulous growers sprayed the silicate-based additive used in road paintmarkings for reflectivity, as when applied to the buds it made it look like they were of incredibly high quality.

This was not scaremongering, I bought some myself. Twice.

I have connections with people in the old synthetic cannabinoid industry and know how cheap that shit is to buy and how easy it is to order from China, so I don't see in any way it being a stretch of the imagination to envisage people growing poor quality weed and adding to it.

I’m not sure the distinction matters to lots of customers. “Synthetic cannabinoids” as a term suggests to customers that they’re buying something like synthetic cannabis, even if it has significantly different effects, and even if it’s chemically nothing like actual cannabinoids. The only regulation (at least in parts of the US where I’ve looked into it) is banning them where possible, but like whack-a-mole new ones spring up, with new compounds and the same implication that it’s ~weed.

Where I live in Germany it may not be the norm but I also came across it several times. Nasty stuff if you suspect something different

Sounds like a dispute over facts. Bring URLs to defend your case.

Hard to prove a negative. It’s not like this is prevalent enough that there’d be large scale testing of weed going on.