When I write (including right now in this comment), I am choosing words in a way that will hopefully be pleasant and easy to understand to a large audience. I think design is like that. I would not use a sentence structure that is distasteful to me personally, or so idiosyncratic that only I appreciate it. You might say I am writing for an imagined reader that shares my tastes, to some extent, but doesn't know all the same things that I know.

If I were literally the only audience for some writing, it would look like my personal notes.

I could be a core part of my audience, or farther from it. I consider myself to be part of the audience for what I'm writing right now, since the words are coming to me as I'm writing. If I'm telling you some information I already know, or documenting some code, then a future version of me that has forgotten the information could be part of the audience. In some cases--and I presume it would be even more cases if I were a professional writer--I am farther from being part of the audience. Then it's important to know my audience well enough to be able to put myself in their shoes a bit. For example, if I were going to write a lesson teaching programming or calculus to someone from scratch, I would need to understand what points my audience needs broken down and explained. I might need experience as a teacher of these topics to do that. As another example, if I am writing to an audience that has different life experiences and privilege from me, and I don't take that into account, my message might land awkwardly or fall flat.

The bigger the creative undertaking, the more important it is that you are bringing something into the world that you, personally, very much want to see in the world based on your personal desires and preferences, in my opinion. If you are going to write a novel, or pitch a TV show, or open a restaurant, it's got to be your thing. Specific and personal is good. You have to keep a somewhat general audience in mind, though. For example, if you are very tall, don't open a restaurant that is only comfortable to sit in for very tall people.

Does this apply to start-ups? It can. Not every aspect of a start-up is a creative work. Sometimes the code is the creative work, and the business idea is what pays the bills. Sometimes the business is where the passion is, and the software is a means to an end. Ideally, different people on a team bring their creativity and passion to different aspects of the product. However, a very driven founder may just want to see his or her idea exist in some form, and not find, cultivate, and encourage a good designer's creativity. Sometimes "design" just means making a whole lot of product and aesthetic decisions that need to be made, and it's not based on anyone's coherent creative vision. When there are a lot of decisions to make and seemingly no objective basis for making them, and no designer that has the team's trust and respect (giving them earned power to make decisions), start-ups just ship things and see if people like them, using "user feedback" as the missing objectivity.

I don't know what designers mean when they say to each other, "You aren't designing for yourself." I have been on projects where I had to remind myself that I wasn't in the target market, though, and it sucked. For example, working on a tool for programmers, and being a programmer, but it not being the kind of tool I would want to use. I am a very creative person, and I am best suited to thinking of my work as undertaking a big creative project.

I think PG thinks of start-ups in terms of hacking, which he relates to painting (famously) and to some extent writing, emphasizing individual creativity and craftsmanship, so it's a similar sentiment.