The example given

is Mount Rushmore. What if mankind died out and aliens observed Mount Rushmore. Would they consider it designed? Why?

Here's what Dembski had to say:

The evidence for Mount Rushmore’s design is direct—eyewitnesses
saw the sculptor Gutzon Borglum spend the better part of his life designing and fashioning this structure. But what if there were no direct evidence for Mount Rushmore’s design? Suppose humans went extinct and aliens, visiting the Earth, discovered Mount Rushmore in substantially the same condition as now.

In that case, what about this rock formation would provide convincing circumstantial evidence that it was due to a designing intelligence and not merely to wind and erosion? Designed objects like Mount Rushmore exhibit characteristic features or patterns that point to an intelligence. Such
features or patterns constitute signs of intelligence.