Sure - I don't doubt that Baku made some people fabulously rich, I'm more curious about "most modern billionaires", specifically "most". I'm not even convinced that most modern billionaires made their billions through petroleum, much less Baku.
In this case, the Rothschilds were already a powerful European banking dynasty. There are many shadowy speculations about the different branches of this family. Perhaps this is where the confusion originates.
The Nobel family did better relative to their previous station. In the end the Baku concerns were sold to Rockefeller before being nationalized after the Russian Revolution.
