The "Oligarchs" were already in place before Putin took power. Effectively a gangster legacy of the Yeltsin years that had by hook or by crook, taken over massive state resources for their own benefit. The notion that they were "based on the most reactionary traditions of Russian nationalism." seems a most unlikely ideological take for what were essentially a "Comprador" capitalist elite who as such of course not only won the approval of the western powers but were welcomed with their dirty billions into western capitalist elite circles with open arms.
As such they most certainly didn't represent "Russian Nationalism" and indeed as soon as Putin (who could indeed be said to do) took power they became his first target as he started clawing major assets and revenues back within the control of the state.
The idea that they are all "Putins oligarchs" is of more recent vintage: I suppose it makes it easier for the West to steal their billions of overseas assets if they can be seem as linked with the Putin regime.

-Of course that why we are also fed so much junk on this subject. If anyone has solid online references on this it would be appreciated.