I feel like you might be conflating a couple of things, though I'm not a DS so could be off base here. My reading of the OP's description is that the vendors w

I feel like you might be conflating a couple of things, though I'm not a DS so could be off base here. My reading of the OP's description is that the vendors were offering interpolative predictions, but did not use a test/train split of data. This is in contrast to extrapolative predictions which I would call out-of-sample. Thus due to not using a test/train split, they achieved extremely good accuracy because they were testing on the same data they trained on. Even though this is "in-sample", you can't use the same data for testing and training.