It's possible he is lying, but who knows? However, there were casting problems for that role. It wouldn't surprise me that they would want an unknown. Besides, the name SHREK would sell with or without an above-the-title star.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle