A regime falls because its enemies are stronger than its defenders.
That's a bit like saying that a regime loses because its enemies win.
The context of the quote cannot be forgotten. It is dealing with the problem of Russian governance in the early twentieth century, but it could equally be dealing with France in the late 18th century, China of the late 19th century or the Soviet Union and Eastern Europe in 1989 and 1990. It each case, the system had deteriorated to an almost senile state. Thus, each was relatively easy to topple.
Compare that to China and Tiananmen Square 1989.
Tikhomirov's statement somewhat reminds me of the oft heard observation that America can never be conquered by a foreign power, rather it could only be brought down by an internal enemy. Such a state of affairs could not happen if the ruling elites maintained the standards of the Constitution.
As the 1917 Revolution, Kotkin points out several facts which contradict the idea that the Russian Imperial Army was so bad. The Russians handed the Austro-Hungarian forces some devastating defeats. The Russian Imperial Army fared much better against Ottoman forces than did the Brits. The Russian Imperial Army took more POWs than the French and Brits combined. Yes they lost to the Germans, especially at Tannenberg, but both the Brits and French also took some terrible thrashings from the Germans.
From what I can gather, Kotkin's opinion is that the 1917 Revolution came about (mainly) because of the contradictions in Tsarism and modern governance which resulted in a total lack of a legitimate governmental structure on which a new government could be built. It helped that Nicholas II was completely unfit for his position, but Kotkin believes Nicholas' father Alexander could not have done much better. The tradition was the Autocrat held all power in his hands and that every area of government was his to command. Political parties were not accepted, even those which supported autocracy.