If they used Twitter or some other platform then they are still reliant on an externally funded system they have no control over. It's not really a backup plan at that point.
Also I don't think you can build a community on Twitter anyway. It's just shouting into the void. They could use it for announcements (and do, LTT is on Twitter, too), but little else.
> Linus of the Tech Tips variety has said that a major reason they kept paying for the forums ... is so that they could have a direct line of communication to their core fan base no matter what happens.
Using it for announcements is precisely the primary reason they claimed to want to keep the forums. So in this case, Twitter would be appropriate.
Also I don't think you can build a community on Twitter anyway. It's just shouting into the void. They could use it for announcements (and do, LTT is on Twitter, too), but little else.