The link you mentioned contains NOT a firmware update. It's just a dump of actual firmware from someone camera. It's not intended to put it back, that why your camera refuses it.
But there should be no differences between SD700 and IXUS800 in terms of firmware. At least, for pairs SD500(IXUS 700) | SD400 (IXUS50) the official Canon's update is the same file for each couple.