It's an interesting question--when is a tree a penjing instead of a bonsai? Single tree penjing would be called bonsai by almost all hobbyists in the USA unless they were told differently.
Yeah I know what you're saying but if anyone were to Google pics of penjing and look at how much different a single penjing tree looks compared to a single manicured bonsai one would have to say there is a difference between the two, in some cases not much but still a difference all the same.
But then again it's all in a word, Bonsai- Penjing, just different cultures with different way of doing things. I also know that in most cases Penjing does attribute itself to a landscape type setup involving more than one tree.