It’s very nearly a synonym for value extraction. I guess specifically it refers to the consequences of value extraction on the web, but really I think Doctorow’s whole goal with that campaign was to make people aware of how awful venture capital and value extraction really are for us. I’m less concerned with preserving a canonical definition of enshittification as I am getting people aware of the consequences of capitalist financialization. As a well-published author himself, I’m sure Doctorow is well prepared for figurative death of the author :P