I think you're probably correct. They had a press release in 2004 talking about "opening" the format. It's a bit of a let down to tell the truth.
"In the past, major Developers such as Steinberg and Emagic applied for and obtained a license to support REX playback in their applications. Now as an open format, any manufacturer, large or small, can support REX playback in their applications.
Third-party manufacturers are encouraged to download REX2 developer documentation. Implementing the Propellerheads REX2 file format in other applications or hardware is free of charge. Further information about the REX2 file format is available at http://www.propellerheads.se/developer"
"In the past, major Developers such as Steinberg and Emagic applied for and obtained a license to support REX playback in their applications. Now as an open format, any manufacturer, large or small, can support REX playback in their applications.
Third-party manufacturers are encouraged to download REX2 developer documentation. Implementing the Propellerheads REX2 file format in other applications or hardware is free of charge. Further information about the REX2 file format is available at http://www.propellerheads.se/developer"
https://www.reasonstudios.com/press/21-propellerhead-softwar...
A bit more info at https://www.reasonstudios.com/developer/index.cfm?fuseaction...
EDIT: Speeling