They didn't overachieve. Their for-and-against was comparable to every other finals team except Melbourne and Brisbane (who underachieved in the finals). The Storm were the dominant team that year. They would have belted anyone:
2017 finals team | Points differential |
---|
Melbourne | +297 |
Brisbane | +164 |
Sydney | +72 |
Cronulla | +69 |
Penrith | +45 |
Manly | +40 |
Parramatta | +39 |
North Queensland | +24 |
As you can see, across the most granular metric (points differential) it was daylight second and fourth. It should really have been Melbourne belting Brisbane in the grand final. The final six teams were all in the same class. The Storm should've smashed any of them.