The man who donated the sperm was heterozygous, which would mean about half the sperm would contain the mutant. You can also see this in their figure 2: >"Sequ

The man who donated the sperm was heterozygous, which would mean about half the sperm would contain the mutant. You can also see this in their figure 2: >"Sequencing of 83 individual blastomeres collected from 19 control embryos revealed that 9 (47.4%) were homozygous wild type (MYBPC3WT/WT) and 10 (52.6%) were heterozygous, carrying the wild-type maternal and mutant paternal alleles (MYBPC3WT/ΔGAGT) (Fig. 2a and Supplementary Table 2). This distribution was expected and confirms that the heterozygous patient sperm sample contained equal numbers of wild-type and mutant spermatozoa with similar motility and fertilization efficiency."