Not sure if I understand this correctly:
Cloud capital is fundamentally different because it can be copied infinitely like a digital copy and requires exponentially less labor. This is different from owning and operating a factory (requires labor) or owning or renting land (limited). There might still be labor but it's insignificant and it scales almost infinitely. That is why it's rent on capitalism. It's this infinite scalability that is the difference.
You could develop a perfect alternative to amazon, ebay, paypal, facebook or google, messaging apps, app stores: The cloud capital isn't in the software or the servers, it's in the account network. Everyone can theoretically move services but that incurs costs by loosing your connections in the network. And marketing makes it so that the majority of people won't do it. So once the network is established it extracts wealth without being productive.
Another example would be paypal - they take something like 1.5% to 3% of all online transactions, practically a tax on all consumer transactions.
And you use digital services all the time and are increasingly dependent on them. And they extract wealth without them having to produce content or advertising, they just rent their ad spaces.
AI generation and robotics will become much more advanced in the coming decades. This too could become cloud capital and wealth extraction without requiring significant labor. Especially once we reach "robots building robots".
I'm also seeing a power grab for the means of generation coming. If public learning data has to be licensed especially for AI to learn from them, it will mean that the cloud capitalists will own the generative AIs. In their anger about big AI "stealing muh data", people will help that IP law will change so that serfs will not be able to use generative AI freely. Maybe LLMs and generative AI are over hyped but they've come a long way in just a few years, in one or two decades it could be incredible cloud capital that extracts wealth to no end, without any labor involved at all.
The feudalism bit I'm not sure about - presumably the scale of social media creates not just cloud capital and massive rents, it also creates political power. By subtly shaping how algorithms present content, we are being ruled by algorithms. The content we see makes us think certain things, argue about this and that and distracts us. MAGA is an example of a sub population mostly disconnected from reality and trapped in a prison for the mind. This is political power similar to a serf under a lord. Not sure how well that relates.
He also mentions the global financial system, I imagine that globalization and increasing refinement of "financial products" also serve as a form of cloud capital that is no longer really tethered to real labor. It's more and more speculative. The central bank constantly prints money and injects it and somehow it ends up in the hands of the cloud capitalists? But that existed before.
Not sure if patents or IP fit into this. And again, not sure if I understand any of this correctly or if I buy any of this. But it's certainly possible that the scale of certain forms of capital create completely different emergent structures. Even if it's fundamentally the same mechanisms, technology might turn it into a completely different beast.
He definitely didn't do a great job explaining this - presumably Yanis Varoufakis wants us to buy his book. That old capitalist trick!