The "don't sell them.. to other developers" bit conflicts with the terms of CC-By.  If the author means that to be an actual license condition, they should probably consider a different license.  Nothing wrong with requesting that as a courtesy, but CC-By makes it technically unenforceable.

Would that not be an exclusion, similar to a DRM exclusion?

As I understand it, a DRM exclusion is waiving an existing restriction of CC-By, whereas this would be adding a new restriction, which CC-By does not permit.