I can't really comment on the historical notes there about the dynamics of financial and industrial capital, but I definitely agree with you that none of those digital monopolies he cited could be classified mainly as industrial capital. Except maybe Wallmart, which is not digital (AFAIK), but is still mostly just a consumer-facing monopsony.

A few (possibly wrong) ideas here, but I believe that the existence of software has created the perfect conditions for rent-seeking, since software is incredibly cheap to reproduce and distribute, but can't effect change in the material world without expensive hardware or humans. Because of that, a corporation can consolidate a Doctorow's Chokepoint in a new market at light-speed, but the actual productive (and expensive) work-force mostly exists outside their employ.

And since the average worker doesn't have enough money for that much rent, the corporations make the most of it cannibalising smaller companies, with shit like Windows licenses for companies, paying to have your posts be read by your followers on Metabook, Google taking a massive cut on apps payments, or stores on Amazon having to pay to have their products appear when searched.

I'm not sure why Hudson, Doctorow and Varoufakis keep calling this "feudalism" but I don't think they're fully wrong in drawing a distinction between those two sections of the bourgeoisie, even if it can't be exploited for revolution.