IVF is exactly how one would implant these embryo "models," and that's why I brought it up in my first comment.
Here's an example of how IVF embryos are graded at day 6, prior to implantation:
https://flo.health/getting-pregnant/trouble-conceiving/ferti...
I think you missed the fact that these models start at the equivalent of day 7, so we're right in the same time window of implantation. Unless you are claiming that exactly at 6 days after fertilization is when every single embryo implants, and at 7 days it is simply too late. But your whole point seems to be that the window is hard to observe, so I doubt that's what you mean.
At any rate, I still see nothing to indicate that these "models" are not simply embryos. I think they are intentionally calling them models so that they can skirt the legal requirements about embryo research.
Unless of course you can point me to something that is actually different about one of these models vs an embryo. The whole article is about how they are the same.