Your perception is correct

. Personally believe
THIS part is honest connotation of true Yamadori. Not seedling nor stump nor any domesticated yard tree. True paid Yamadori collectors collect/sell whole trees AFAIK. However deciduous trees likely will be out of leaf.
I think there is an esoteric case to be made for including the rescuing very young, nature-made saplings from unusual environments as "yamadori," but I would otherwise be inclined to agree with you. Further, I additionally suspect that many Japanese would likely disagree with my willingness to classify urban sapling collection as yamadori.
My case for this is purely etymological. The word "Yamadori" [山採り / (No equivalent)] seems to be derived from
"yama" [山 /
Mountain] and
"tori" [採り/ Picking] or
"torimasu" [取ります /
Take.] So not as to confuse bonsaists by referring to the art of collecting trees from the woods as **"Mountain bird" we thusly arrive at yamadori (lit: "Mountain picking/ taking") as we know it. I think a Japanese person would probably be confused if you said "I practice yamadori" and then showed them a picture of a yard-stump. They might even think, "In America, walking to the backyard is the same as going on a hike in the mountains...
I see. What a uniquely American problem..."
None of this is to suggest that yard-stumps/ domesticated yard trees can't eventually make for lovely bonsai. Further, I certainly don't mean to suggest that we in the West should be introducing any new element of elitism into the art and hobby of raising little trees. That all being said, I would argue that this is something which is distinctly different from "yamadori." Not better or worse necessarily, just different. Because this is an internationally-shared hobby, I think it would be best if we called it something else to avoid confusion.
Maybe we can call it "stumping"? There are already several definitions for this English word, so I don't see a problem with adding one more.
"Kiribakudori" [切り株採り / Stump-picking] would be the most appropriate word to use for this practice, in my opinion.
DISCLAIMER: I am not a Japanese expert, but I am studying the language and enjoy etymology as a hobby.
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Notes:
**
"Yamatori" is a probable spoken shorthand of "yama no tori," which literally translates as "Bird of the mountain."