The CDDL has a copyleft provision, similar to the MPL. One of the clauses states "The Modifications that You create or to which You contribute are governed by the terms of this License."

The CDDL is file-based, just like the MPL that it was forked from, and was specifically authored to allow mixing with proprietary code. The only thing that the part you quoted means is that changes to existing files must be available under CDDL terms (or freer).

In any case, the contributors who own the rights to the code they submit are free to make it available under any other license they want, whether it be MIT or something else.