When a user adopts your product they're saying they think it's special.

That is, it's worth exerting energy to adopt.

When some meaningful subset of people think your product is special then you have PMF.

If someone who has a vested interest in the thing (e.g. investor, boss, someone whose day job is working on it) says it's special you can't tell if it's actually special.

The mark of it being special is entirely people without a vested interest taking actions that demonstrate they think it's special.

Everyone thinks their thing is more special than it actually is.