It's not about whether Voyager can destroy a cube or not.
Chipping away at the external hull was useless because there wouldn't have been enough time to do any decisive damage. It was a battle of attrition. Starfleet's entire force was being picked off, one by one. Yes - Hopeless.
And we must accept that Voyager, a single starship, can dance around like a fly and survive attacks from these very same cubes. Or that Seven's parents in their little boat can walk right up to Borg Central and spy on them...
Now we can dream up all kinds of explanations for these things until the cows come home, but at the end of the day it doesn't negate the statement: "Voyager weakened the Borg." It just updates it to: "Voyager weakened the Borg, and then I used my imagination to strengthen them again."