Usefulness is about the general usefulness of a project, not design.
UX and DX ( you removed halve), it can be an API. An API doesn't need design or UX.
There's not a single absolute requirement on design.
Since product design/ux is about graphics/visuals.
Please check the parent link for the "Judging Criteria" section (two of the three strands are Usefulness and UX) and "Project Ideas" section (literally all product designs).