Which honestly feels a bit silly to me, because the Chinese penjing community are also obsessed with the details of the craft - may be to a slightly less degree than their Japanese counterpart due to the Japan culture's tendency to formalize things. But I promise you penjing enthusiasts are just as likely as bonsai enthusiasts to object to stick-in-a-pot being called a penjing.

(eg of a penjing site full of guides and discussion about the type of plant, the soil, the pot etc: https://www.pjcn.org/ )

And just like the article says about Japanese and their attitude towards the "not bonsai" - outside of the enthusiast communities, most people don't care, and will enjoy their small-tree-like-plant and call it bonsai/penjing anyway.

So I don't see why one would object to calling a casual mini tree a bonsai but is comfortable calling it penjing. Either both are okay or neither is.

I agree with you. My mom is very into plants but she never calls her pots of mini trees penjing. But I guess there's the language aspect of the word usage as well: 'penjing' (literally pot of scenery) sounds very formal like written language, not very colloquial. It would sound like an advertising word as if from a production video or a market. Whereas the Japanese counterpart is regarded as an established high art/profession (which is common in Japanese society).