> Despite having less pre-filled suares, our Hard sudokus aren't harder than Arto's puzzle since they have more than one solution, which makes them significantly easier to solve than sudoku puzzles that have a single solution.

I would say that makes them invalid. Valid sudoku puzzles should have unique solutions.

I agree. And running Arto's puzzle thru a solver, it has more than 10 solutions, so they lied about that too.