I don't think Paul Graham meant ramble, use "uhs" and "ums" and and expletives as you and other commenters have said.

I think the point was to use common and simple wording so that people can understand without doing a lot of work. People have a lot more practice understanding common and simple words. So it's much easier for them to read, no matter how smart they are.

I think this was pretty clear in PG's essay, but the headline probably threw people off. Maybe he should have said it differently, such as "Make your writing as simple as your speech."