Lisp doesn't have such a requirement and does not have 'ISeqs'.
> I and many others disagree,
That does not make a convincing argument, since you have never used an actual Lisp - as you recently said.
> including Clojure's designers, who designed it as a Lisp.
Derived from Lisp mostly as a blend of FP, Java hosting/integration and Lisp ideas.
> Homoiconicity, macros, S-expressions and FP make a language a Lisp.
Lists are deprecated in Clojure (for maps, sets, vectors, ...), No interpreter, no Lisp down to the metal, no Lisp in Lisp, no images, core Lisp data structures look different, different API, different syntax, impoverished REPL, strange numerics, Java leaking in many places, lots of compromises because of implementing it on top of a not-Lisp-friendly VM, ...
Lisp derived, but Lisp looks & feels a bit different.
Clojure requires that the second element in the pair be an ISeq.
> since it is mostly incompatible and lacks a lot of the usual Lisp features.
I and many others disagree, including Clojure's designers, who designed it as a Lisp. Homoiconicity, macros, S-expressions and FP make a language a Lisp. OCaml and SML are about as different as Scheme and Clojure, yet no one thinks they're not both MLs.