It's a subjective opinion based on plenty objective info. Path widths are objective for how large you will need it to hold guests. Weenies and sightlines are objective data. Counting the times you read wit in the shop descriptions is objectively measurable. Placement of restrooms can be inferred based on capacity needs. Land transitions are also logical; Star Wars Land does not transition well into Toy Story Land... with Narnia and Nightmare, you have snow and Christmas to transition them. Also, using other parks as a baseline, you can figure out realism if you base it off that logically with rough measurements, etc.
The end result is subjective, but it's not just all 'opinions'. If the judges are going after what they prefer, yes you are 100% right, but if an experienced judge is critiquing a singer's voice, it's not an exact since, but using logic they can get a selection that is far better than just a random person singing...
Creativity can also be objective too. Has anyone done the same execution as that for Narnia before?
Judges are supposed to be more impartial, not subjecting themselves to emotions.
You cannot objectively state an order of which one is the best, but anyone logical, that bases their consensous on knowledge of themed design would very likely come to a different result