Her forward impeller nodes had failed completely within seconds of Sirius's destruction, and this time there had been no way to repair them. Worse, her after impeller ring had gone for over forty-five minutes, as well—three-quarters of an hour in which she had coasted another ninety-four million kilometers outward while damage reports flooded in to her airless bridge.
Note the text--"this time there had been no way to repair them." Also note the timing. The grav lance fired, the energy torpedoes tore through the Sirius, the Sirius blew up, and then--seconds later--the impeller rings failed. And this was after significant damage which had already taken out the forward impeller for a while. The impellers were running on patchwork battlefield repairs, and Honor had been surprised that the forward impellers had been repaired for as long as they were. The ship was barely alive.
It was battlefield damage that took out the Fearless impellers, not the grav lance.