They are the same until you apply a test voltage to them.

Then 0v will start to sink current and your test voltage will be 0v. Open circuit won't sink any current and the reading will be your test voltage i.e. 12v.

The empeg probably detects falling or rising voltage like manchester coding rather than actual logic state so by adding a 'pull-up' resistor, you are applying a test voltage which makes your output either 12v or 0v which the empeg needs to detect the state change.

Oli.