Yes and no. The problem is that there is some truth to that but at the same time it ignores a larger problem with the French army, or rather it's commander.

By 1812, the French army had grown greatly. With contingents drawn from every defeated Kingdom, Napoleon set to defeat Russia and bring them back into the Continental System with an army of almost half a million men, the largest that had been seen in Europe until the Great War.

Before this, the defeated enemies of Napoleon were hard at work at reforming. The Prussians developed War Games that are more akin to our table top wargames with specific rules to reflect warfare with the explicit purpose in training officers with a lack of men (as the French/Napoleon limited the Prussian army to around sixty thousand men). In combination with the reforms of Scharnhorst, a Prussian officer, Prussia would emerge from the ashes and lead the War of the Sixth Coalition, also known as the War of Liberation.

Austria also focused on reform but started much earlier after their defeat in 1805. Archduke Charles was given wide room to reform the Austrian army, trying to recreate the French army in the Austrian system with a corps system that worked ineffectually only due to the poor training of older commanders. The Austrian reforms would continue after the defeat of Austria in 1809 and be available for Schwarzenberg in 1813.

Finally, the Swedish king adopted a son, Jean-Baptiste Bernadotte, a Marshal of France and one of Napoleon's least effective generals. However with Bernadotte comes an understanding of how Napoleon works. When Bernadotte, as Crown Prince, joins the Sixth Coalition to fight Napoleon, he tells everyone how to defeat Napoleon.

This is where Napoleon is at fault. Napoleon as a commander was absolute as he was in his reign. The greatest criticism of Napoleon is that he was a skilled commander but only he could command. He didn't teach his Marshals how to command and rarely gave them independence to spread their wings with very rare exceptions. Even Marshals that were highly gifted in command (such as Marshal Davout) would be restrained by Napoleon. As such, the French army had only a handful of skilled commanders that could fight on their own.

Bernadotte knew this, he spilled the secrets of Napoleon and told the Allies to separate the French army, destroy the corps and prevent them from uniting under Napoleon. This only aggravates the problem of the single commander, as the French army grows to almost a quarter of a million. It's too big for a single commander to lead but also works against Napoleon as he puts Marshals Ney and Oudinot (the former Napoleon considered to immoral to command anything more than ten thousand men and the latter begged Napoleon to never be put in an independent command as he was a self admitted mediocre commander) in command rather than Marshal Davout whom he sent to Hamburg to hold (which he did from early 1813 till mid 1814 under constant siege).

Finally, warfare had changed in the short time Napoleon was a commander. Armies were much smaller in 1805 than in 1813 with the battles being no more than an eighth of a million strong with both sides. This allows Napoleon to work on movement and maneuver rather than brute strength, which is what he did in 1812/13. Head on attacks and massed artillery would be important in those years rather than the speed and movement that would come to exemplify Napoleonic Warfare. Ironically, Napoleon would return to this movement based warfare in 1814 when he had a smaller and more skilled army based around his Imperial Guard rather than a massed army.

In short, his enemies grew to know him but also pushed him to the limit.