Is (Lieven, Sokolov) the reference you are referring to? If so, please provide the title so I can at least take a look at it.
Russia 1812 was a key element in the eventual defeat of N.; but 1812 was only the beginning of the end.
Note: Perhaps we have a definition issue here; for an event to be considered "decisive," it must make all following events irrelevant.
Lützen, Bautzen, Leipzig, Waterloo were anything but irrelevant events. N.'s under-horsed cavalry and his Marie-Louise did pretty darn well, especially with their dramatic victories at Lützen and Bautzen.
Perhaps the most important result of Russia 1812 was that it decisively proved to everyone that N. really could be beaten. The pitiful sight of the Grande Armée returning finally cut through all the propaganda from both sides and even the peasants could see with their own eyes and understand what it meant; this could finally be the beginning of the end.
Now imagine no English gold is forthcoming, the Russian Army holds up at the Nieman and figuring -since they were broke- the best bet would be to go home knowing well and good that naughty Boney would never try another round with the Russian winter. What then happens when N. returns in force early in 1813 and the only army it has to face are the Prussians and the Landwehr? N. retakes Berlin, declares a free Polish Nation as a buffer against Russia, Austria is pretty hopeless without Prussia and Russia, Metternich probably cuts a peace deal giving N. his German territories with Austria getting Italy, only Wellington is left.
Even after Russia 1812, 1813-1814 still took the full weight of the Russian, Austrian, Prussian armies working in concert, all receiving considerable financial support from the Brits, to finish the job.... temporality. For me, the army that N. was able to build in 1813, and what he was able to do with it, is final proof of his genius organization abilities.
For 1812 Russia to have been truly decisive, the Russian Army needed to finish the job; but, instead, N. still lived and escaped to start all over again. If N. had died in Russia as he should of, how many 100s of thousands of lives would have been saved? Too many to even consider.