This is like saying that a language with casts doesn't have a type system. No, it has a type system, it just also has an escape hatch that lets you break out from the type system when you need to. Lisps' functional-ness is generally the same sort of thing: They are designed primarily around function-oriented programming, but provide straightforward escape hatches into imperative programming. Common Lisp tends more towards being multi-paradigm than Clojure (i.e. provides more imperative accommodations), but both have strict evaluation, which the author seems to equate with imperative-ness.
The author spends more time talking about a small core and non-leaky abstractions than about imperative-ness. On these points, I totally agree and think it's a well-written article. I don't exactly see the connection with being imperative, and I think it's a shame that the title will draw attention away from those points. The thing that I really like about working in Clojure is how the abstractions are built with care: I feel like whenever I just use the base building blocks, they fit together seamlessly and edge cases usually don't come up.