I used to be a musician.
But before you were a musician, you were not a musician.
Even from the very beginning me (and my teachers) would say "piece of music/art"
That piece was a piece of art or a piece of music. What you were playing, the first time you played notes, probably wasn't music.
You'd say "this painting" while you're making it, not "this piece of paper that will become a painting when I finish it"
I think you would say 'I'm making a painting', not 'this is a painting'.
The rest about "that isn't a shallow tray" argument, I would genuinely like to know why this is not a shallow tray while those examples are.
Some of the examples actually are on (very) shallow trays. Some of them are not bonsai, but another art form. But I have to admit you are right. I gave you a too generic definition. In general, bonsai are trees in shallow trays, but that can be slabs, rocks or anything that looks good and gives a natural impression of a tree in its surroundings.