They do not need everyone to stop using 0.8, just a majority (so that the 0.7 chain becomes the strongest).

You would need to get everyone to stop using 0.7 (or much more than a majority, or as soon as another block of this kind resurfaces the clients would be useless again).

0.8 accepts blocks that 0.7 rejects, 0.8 does not reject any blocks that 0.7 accepts. So 0.7 is more compatible going forward until the problem is solved with 0.9 etc.