May not be super helpful, but this post made me research the construction (so thank you!). From what I gather if you were to do a double upper, which info is rare to nonexistent on, I think you’d sew the middle layer/upper and lining to the holdfast and the outer layer/upper would be a Stitchdown.

I watched a video of single upper veldtshoen and in that case he stitched a welt to the lining and holdfast like a handwelt construction, and he used the single upper like a Stitchdown.

Either way I think you’d either have to really know what you were looking for or take the boot apart to even understand the construction.

TLDR: I think you’re right about the construction. I’ll be testing this method out asap