I thought Lenat was pretty up front about what Eurisko did and why it was nearly a one-trick pony. What I gathered from his writings was that he essentially used a Genetic Algorithm (which was novel at the time as it would have been a very early, possibly the first, implementation) to modify the Lisp code. So it was essentially randomly swapping out tiny snippets (i.e. Lisp atoms) of the code and managed to achieve good results because of the impedance match between the Lisp code and the problem domain.
So while I wouldn't say it was smoke and mirrors, Eurisko was both novel (for the GA approach) and disappointing (it had nothing resembling understanding of the problem) at the same time.