I think I've figured out my issue with 'just write' being advised to readily:
You can't "just write" until you've made a decision about what to write, even if it's just the next sentence.
On one extreme, complete pansters make decisions about what to write sentence by sentence, paragraph by paragraph, scene by scene.
On the other extreme, complete plotters make as many decisions as possible before they start writing. When they do start writing, they're only making small scale decisions, like how to phrase dialogue or describe specific actions.
When a writer makes the wrong decisions on their first draft, which is inevitable, they have to figure out what the mistakes were and fix them. The more bad decisions in the first draft, the more rewriting.
I think those who say "just write" is a valid solution for everyone aren't considering that the decisions a writer is struggling with still have to be made. In my experience, when you're forcing yourself to "just write", you're making decisions as fast as possible so you can keep writing.
Sometimes that's exactly what needed to happen. Other times, the stress a writer felt about a decision was justified, and they may have caused significant problems by choosing too quickly.
People like me simply can't handle writing and making big decisions about what to write at the same time. Sure, I can do it if I absolutely have to. But every panster who insists that they lose interest in a story if they outline it could probably write that way if they absolutely had to.
Good advice for a plotter? Unlikely.
The OP in one thread was worried about their ability to handle a complex three part story. They were in the process of making an outline, which would be particularly useful because there were going to be overlapping timelines. There are plenty of ways to give advice on the OP's problem without saying, "Just start writing, even if you haven't finished your outline."
The other OP, who didn't know what to do with their inciting incident, looked like they needed to learn how to brainstorm. Later in the thread they realized they needed to write down everything they knew about the story already, and work from there, which is often how brainstorming ends up starting.
In either case, if those OPs worked best with a complete outline and they hadn't finished one, "just write" would have been useless advice. Was that necessarily the case? I don't know. But you don't know either.
Telling a plotter to "just write" before they're done with a plot outline isn't just useless advice; it can be harmful advice. You can't just make the assumption that everyone's process is the same as your own, and anyone putting off writing is doing it for no good reason.