Feudalism is a rigid class structure determined by blood inheritance. That's not really what we have. A noble who looses their money would still have the privileges and rights granted by their title - they would still be part of the ruling class. If a member of our ruling class were to loose their money, loose their source of passive income, they'd very quickly no longer be part of the ruling class.
I haven't read Varoufakis' book but I can only assume it completely misuses the term feudalism and tries to make a case for "ethical capitalism" (or overtly or by implication), as though you can point to a particular period of capitalism when everything was great and the US wasn't dropping incendiaries or napalm on kids in Asia
Varoufakis is a commie so the book doesn’t make a case for “ethical capitalism”, no. Not sure if he is a Marxist, but I’m pretty sure he has called for a revolution many times.
Edit: I think his point is not so much that we’re at a completely different system, but that the current mutation of capitalism is so far removed from what people think of as capitalism that we need new language to talk about it.