I'm not sure why he feels the need to describe the embryo if the medication he's pushing does not effect implantation, but more importantly, he is simply wrong. There is a number of instances in the literature in which low-dose anti-progesterone meds have shown to have a deleterious effect on the uterine lining, thus effecting implantation. All of my sources for this series will be from the pro-abort and contraceptive journals, and most of them are available in their entirety online. Here is an article from 2005 describing the effects of anti-progesterone medications, including ru-486. Ulipristal was not included in this paper, but is in the same category as the other anti-progesterones mentioned. Under the heading "contraception" as a possible future use for these meds, the authors state this:
Archer says there's no evidence that ella can interrupt an existing pregnancy or prevent implantation, and other experts point to the drug's 2 percent failure rate as proof. "At that point, it's just a microscopic ball of about 256 to 550 or 600 cells that will differentiate in the future," explains Archer. "You won't see a head or fingers or any fetal organs."
Mifepristone and other PA also have contraceptive potential (Spitz et al., 1996, 2000; Bygdeman et al., 1999). They may act by several mechanisms. At low doses they inhibit ovulation by blocking the LH surge. Low doses also retard endometrial development by virtue of their antiproliferative action. As a consequence, the endometrium cannot support implantation. (emphasis mine)
Low dose anti-progesterones have definitely shown to have a deleterious effect on the endometrium. Even in the paper that was cited by the Slate article, we have Anna Glasier making this statement:
Progesterone-receptor modulators, including ulipristal acetate, given at high or repeated doses have an effect on endometrial histology and histochemistry that could theoretically impair implantation of a fertilised oocyte.29,30 Although an endometrial effect, and therefore an additional postovulatory mechanism of action, cannot be excluded, the dose of ulipristal acetate used in this trial was specifically titrated for emergency contraception on the basis of inhibition of ovulation and might be too low to inhibit implantation.
Let me explain what she is saying. She knows that at a certain dose there is absolutely no question that Ella has an endometrial effect. The experimenters have attempted to guess a minimal dose in which this effect may not occur. This is why it might not inhibit implantation at that dose. They have no evidence that it doesn't effect implantation - only their guess that it won't at this dose.
What should we go by? Experimental evidence that shows that anti-progesterones have a dose-dependent effect on an embryos ability to implant, or a guess by a researcher that it might not do that at a low enough dose. Anyone claiming that there no evidence of a post-fertilization mechanism for this drug is not basing their assertion on science. They are merely using wishful thinking.
Is it possible that ulipristal at low doses does not effect implantation? Sure - just like many things are theoretically possible. However, in order to show that is the case will require evidence, which is exactly what they do not have.