The phrase is said with good intention and it means well.
My thing is, an audience is an emergent property, not a prescriptive property.
When you write something, people who like your stuff...read it and you have an audience. If you don't have a following you'll promote your content to get more eyeballs and then maybe the interested people who are left are your audience.
The expectation when someone says "build your audience" is to have people waiting before you speak.
Why would anyone listen to me unless I have something valuable to offer.
Understanding who you want to serve, what their problems are, and how you can reach them before writing code is the main intention.
You can reach out via Email/Twitter DMs/Discord groups/Slack groups and tell them about your app.
But, waiting to build your audience before writing code seems like a pipe dream, and what happens if this audience doesn't have the willingness to pay for what you offer?
This is a vague structure IMO:
Sometimes you pick a problem and try to find the audience which is what you want to avoid.
Seems easy in theory, it is incredibly difficult in practice.