From the article: "Still, unless your startup has access to large pools of capital or have a brand name like Katzenberg, Lean still makes sense."
Side projects, solo entrepreneurship are not mutually exclusive and can actually be complementary. Lean is a methodology to validate demand for a product. Solo entrepreneurs can and do benefit greatly from employing it.
When actually, most probaly are not. But this is almost impossible to measure as who can survey everyone with a side project.
Solo entrepreneurs, side-projects, 'lifestyle businesses', ISVs, and other small businesses in tech don't really fit into that category. Although while many of them still call themselves startups (which is fine) the distinction is quite clear.
Lean Startups has fundamental business advice that is good for any company. But Steve's analysis here is clearly targeted as a specific group of traditional high-growth startups which angels/VCs invest in, from which the original idea for Lean was spawned and designed for (and later found wider applications outside of startups - for better or worse).