Maybe. But I think the act of making it public is an important distinction. I don't post to my blog very often (and traffic is essential zero), but when I write I try to make it worth reading. I'm certain I would not take the same care with a diary entry that would never see the light of day.

playing to, and trying to please your audience is the kiss of death, if you write for yourself and forget how you might be seen by those reading you, your writing will improve.

I didn't read it as "playing to... your audience." When I write a public blog post, I put some thought into coherence, logical ordering, etc. and I (at least briefly) proof read the post for massive errors. I wouldn't do that for a diary entry, but it helps me clarify my thoughts.

That might apply if you choose topics purely to please your audience, but remembering that other people could be reading what I'm writing has definitely helped me improve over time. A post I write for myself will inevitably have no structure, coherence, or focus. Writing for others forces me to clarify my thoughts and ideas while viewing them from the perspective of a stranger, which is a skill that extends to all aspects of my life.