Mr. Holstein isn't a MacOS or iOS developer. He comes to WWDC at the invitation of his niece, Clarus. She works for Apple and has been a major figure in their developer community for years and years.
The two have a pretty close bond. Back in the 80's it was a pretty big taboo for a cow to date a dog, so when Mr. H's brother started dating Mabel, it didn't matter that she was a Dalmatian and could almost even "pass"...it caused a bit of a rift. Mr. H was the only one among his siblings who truly welcomed Clarus' mom into the family.
Which isn't to say that he himself didn't have some doubts, early on. Still, he came out for a "meet the family" dinner at Applebees. It was immediately clear to him right then and there that the two were in love. Yes, the couple would face many challenges from a narrow-minded Society, but in the end, love is all that really matters. Mr. H enjoys joking that Mabel is "obviously one of those exotic 'show' breeds of cow."
Mr. H's sisters could never get past their prejudices, and have mostly cut Clarus and her parents off. Oh, well. It's their loss. What a shame, to never know such a fine young woman as Clarus the Dogcow.