I believe the explanation is that Segur was not there when Mortier attempted to carry out his orders but he assumed that a) Mortier had been ordered to destroy the Kremlin, b) Mortier did attempt to destroy the Kremlin, and in at least one version (a Russian account I believe) Mortier then left the scene to catch up with the army and Russians were able to extinguish the fuses, c) Mortier reported mission accomplished as some of the fuses had not been extinguished so he heard explosions but did not visually verify, d) Segur considered Mortier a reliable source.
At least one part of the puzzle is that Segur might more accurately have written "Mortier reported that he had carried out his orders and thus the Kremlin no longer exists."
Bottom line: I don't know if one can as seriously question Segur's account of incidents for which he was a witness, but for any source at any time in history one always has to be attentive to their accounts of any incident at which they were not present: how did they know? who was their source? how did that source know? was that source present?
Robert A. Mosher