The Russian identity is actually quite weak without strong state power, which is what the authors rightly noted. It is in a sense an artificial identity maintained by a strong authoritarian state, which actively promotes nationalism and promotes the propaganda of this identity.
But when ethnic Russians are placed in a democratic foreign environment, even where they speak Russian and are fully connected to the Russian culture, such as in Ukraine, the gravitating force of Russian central government precipitously declines.
They prefer to identify as Ukrainians even if they still regard their native language to be Russian. Since their children are mostly being educated in Ukrainian language, the next generation will identify with Ukraine even more.
Ukrainians are capable of maintaining their identity and even their language for 3-4 generations in a foreign environment like the US or even Russia. Russians dissolve much quicker without a strong state to support their identity. It's much harder to identify with an empire when you live outside of it. Ethnic roots are much easier to identify with. But Russian identity is more than ethnic, it's imperial.
The interesting thing is that many Ukrainian villages in Siberia still speak Ukrainian. It's also interesting that many Russians who still believe Russian is their native language, are nevertheless re-identifying themselves as Ukrainians.
Ukrainians have contributed a lot of manpower to the growth of the Russian empire. Many of the cities in Siberia were started by Ukrainian Kozaks, many of whom still identify as Kozaks. Interestingly enough, their ethnicity was recorded in their passports by the Russin imperial government as a "kozak" ethnicity. Obviously, such an ethnicity doesn't exist. It's simply a replacement for "Ukrainian". Mostly Ukrainian fought in the Russo-Japanese War, The Crimean War, The Finnish War, and so on. Many Ukrainians have been sacrificed in these imperial projects. Although obviously Ukrainians must be good warriors, if they weren't they would not have been used in this way.
Some of the people changing their identification may be ethnically mixed Russian-Ukrainians, who were recorded as Russian in the Soviet times and are now justifiably changing their identification to Ukrainian.
Some other people who have "changed" their identification are people who were recorded as Russians during Soviet times and even had their lasnames changed to look Russian in the 1930s, but are in fact ethnically Ukrainian.
A good example is Gen. Morozov, who knew that his mother was Ukrainian, but had assumed that his father was Russian until he discovered after his father's death that both his parents were Ukrainian.
My own grandfather's lastname was Turkulov, so I thought he was Russian. But I found out that his father's lastname was actually Turkul'. (I have a copy of his Soviet file documenting his arrest and murder by the government.) The -ov was simply added by the Soviet government on Stalin's orders. This was especially the case with all the Muslim people groups - practically all of them had the -ov added to their names.
It is also a known fact that some Ukrainian cosmonauts had their lastnames adjusted to look Russian to an international audience.