At Elex' product page on german Amazon it's already stated and now Michael Rüve from Piranha Bytes also confirms in the official forums that the retail version of Elex will require Steam. While the forum post is only about the Collector's Edition, it's safe to assume the Steam requirement is true for the standard version as well.

With this Elex moves away from the Risen 3 course to offer the disc version DRM-free. If you don't like the Steam requirement, you have the option to buy Elex on DRM-free however.