There are PR agencies that take care of your content to be trending. They do use fake accounts to upvote and manipulate, but you as a customer don’t have to know that.
Wouldn’t it just be easier to ask their team to upvote the post? They probably all have legitimate hacker news accounts already and are happy to upvote their work.
Occam's razor it's more likely the mods or YC has some social or other connection to the project