Here's what the license states:

    You do not sell or otherwise commercialise the Work or Derivative Works as a Game Engine Product.

    "Work" shall mean the work of authorship, whether in Source or Object form,
    made available under the License, as indicated by a copyright notice that is
    included in or attached to the work

    "Derivative Works" shall mean any work, whether in Source or Object form, that
    is based on (or derived from) the Work and for which the editorial revisions,
    annotations, elaborations, or other modifications represent, as a whole, an
    original work of authorship. For the purposes of this License, Derivative Works
    shall not include works that remain
    separable from, or merely link (or bind by name) to the interfaces of, the Work
    and Derivative Works thereof.

If you write your own map editor, it's neither Work nor a Derivative Works so this restriction doesn't apply.