You're right that it's a month to month contract so can cancel.
However, what most have missed is that their calculation is based on getting the first 2 months free. However, the terms of this offer say that it only applies if you do not cancel. While the poster then checked with a 10 month scenario - close to the 12 month handset term anyway - that's vastly different from cancelling 2 months in not having paid anything at all for the usage - and the points. I'd expect Vodafone to apply their terms there and not give the 2 months free if you cancel in two months i.e. the plan cost would then be $160 + 74, so $234 for the two months and 40k points.
This is all stated in their terms so not something that can be disputed either. Some may get lucky - but then with the hordes that have applied for this, that's also less likely given the cost of having to give this out to so many.