The difference is that oligarchs are in the government’s pocket, while the government is in the pocket of billionaires.
I mean the term oligarch has a clear definition that differs from billionaire, but to be honest pretty sure many billionaires meet the criteria…
Quickly from wikipedia
A business oligarch is generally a business magnate who controls sufficient resources to influence national politics.[1][2] A business leader can be considered an oligarch if the following conditions are satisfied:
uses monopolistic tactics to dominate an industry;
possesses sufficient political power to promote their own interests;
controls multiple businesses, which intensively coordinate their activities.Edit: also this smells a bit like tankie propaganda pointed towards liberals and conservatives