I need to produce some 100% SCORM compatible content. I need to do
this with an OSI-certified authoring package. I already know eXe, but
I am wondering whether something more robust already exists. If so I
have not been able to find it so far.

I should either run on windows XP or on Linux.

Consider to author your content in DocBook format - there are number
of OS tools for DocBook authoring. And then convert to SCORM with
something like this: http://pyxx.org/DocBook2SCORM/overview.html