The post is saying that you don't need a cofounder for the subset of ventures that a very technical person can tackle alone. The types of ventures that YC funds are generally not plausible to tackle alone. There is no conflict between PG's ideas and this essay so long as you accept that there are ventures of differing scope, some need more than one founder and some don't, and YC generally invests in the latter.
No serious venture is plausible to tackle alone. But every venture that YC has funded could be done with a single founder. There are other people in companies and they're called employees. You can get to demo day with one founder and employees.
How many YC companies have failed or split or reformed because the founders visions didn't align?
