How did they get sperm if he had a vasectomy? Cells in the semen would not be sperm, but other somatic cells. These could easily contain donor DNA
Was curious myself, sperm still made, it seems: https://www.mydr.com.au/mens-health/vasectomy-frequently-asked-questions
I’m believe that actual sperm would still be manufactured in his testicles. How they are sampled, I wouldn’t know. So the sperm might depend on the blood for the DNA? (For a female, I think that they are born with all of their eggs.)