Florian,
[As far as I know,] the Oryx editor was originally MIT licensed. This codebase went through several big changes already, one of them being closer tied to Signavio (with Signavio core components and an academic initiative) and a license change to GPLv3. Note that we forked our version of the Oryx editor before the license change, so we are still using the old MIT license. The fact that the license file points to a URL, which has since then been updated, is confusing I admit.
Also see:
http://docs.jboss.org/jbpm/v5.4/userguide/ch.getting-stated.html#d0e336
Kris
Disclaimer: I tried to answer this to the best of my knowledge, but I'm not a legal expert.