It looks like there are two steps, though.
My understanding is they humanized the mice first, possibly with fetal DNA (that’s what we don’t know for sure), which gives them a human-like immune system. Unborn babies do start developing an immune system according to this: https://pubmed.ncbi.nlm.nih.gov/6606446/. Otherwise, we would be losing a lot of newborns to covid and I have not heard of that.
THEN they use these humanized mice who are “immunized with a DNA plasmid that expresses SARS-CoV-2 spike protein and then were boosted with a recombinant protein composed of the RBD of SARS-CoV-2 spike protein”. The mice mount an immune response and they use those antibodies and the antibodies from recovered human patients.
We know that the company uses adult and fetal stem cells for their research, we just don’t know for this particular usage.
The jury is out for me.
Except of course, the company states that no fetal tissues were used in the development or production of their covid monoclonal antibodies.
Since they admit to using fetal tissue in other projects and are completely open about using HEK293 for testing, why would they lie about it for this project?
Especially when they are proud of what they do and how they do it and are well aware of the public relations cost of dissembling.