Anything is possible, but the fact is fetal tissue or DNA has no use in Covid treatments or probably any infectious disease treatment because they don’t have an immune system of their own yet. There would be no antibodies
They would have to be getting their b-cell dna from convalescent blood or plasma.
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.