Put another way, do think there is ever any amount of CSS that is not superfluous?
CSS can be non-superfluous in documents, like when formatting text or a table.
I'd like to focus on the fact that this required orders of magnitude less effort than designing a web site, and most importantly – the presentation did not suffer. There were no compromises to be made.
Some have mentioned that this lacks functionality, but I'm having a hard time imagining what that might be. Maybe mobile-readiness, in that it's preformatted. Considering that this is a Unix-y tool for power-users, I think it's ok to expect them to read this on a PC.
I'm all for reducing complexity, but at this extent your are removing pretty huge pieces of functionality for extremely marginal streamlining.
It's a gimmick
HTML isn't a technical requirement, but they do say their goal is to sell the service.