You’d need to test every cell in the embryo to be sure none of them had off-target mutations, and DNA sequencing doesn’t leave the cell alive, so you can’t prove it worked without killing the embryo. He tested some of the cells and discarded embryos where those cells were damaged, but there’s no way to know if the untested cells in the embryos were fine, and given what we know about the reliability, it’s more likely that there are problems than not.
You’d need to test every cell in the embryo to be sure none of them had off-target mutations, and DNA sequencing doesn’t leave the cell alive, so you can’t prove it worked without killing the embryo. He tested some of the cells and discarded embryos where those cells were damaged, but there’s no way to know if the untested cells in the embryos were fine, and given what we know about the reliability, it’s more likely that there are problems than not.