You only need to finish the final orb yeah, technically it's an any% run so the other orbs are just time wasters

To get the demo complete HUD screen you have to complete all objectives in any order. All 4 orbs are objectives.

Completing the final orb on a fresh file (which has the any% category) does not complete all the requirements for the demo, you have to complete all the orbs, pick up all the required weapons and perks before you get the screen showing you that you have finished the demo.

For a geared run there is a category for final orb, this is the category which your goal is to complete the last orb only which means you can go directly to it and doing the other orbs would be time wasters. This would be done on a save which has completed the demo previously.