"Any Covered Software that You distribute or otherwise make available in Executable form must also be made available in Source Code form and that Source Code form must be distributed only under the terms of this License. You must include a copy of this License with every copy of the Source Code form of the Covered Software You distribute or otherwise make available. You must inform recipients of any such Covered Software in Executable form as to how they can obtain such Covered Software in Source Code form in a reasonable manner on or through a medium customarily used for software exchange."
If all freedoms granted by the GPL are ensured downstream by the CDDL, why is it considered incompatible?
Because (a) it does not let you relicense as GPL, which is the only thing GPL compatibility means ("Can you be press-ganged into our army later? Great: you're compatible. We just haven't assimilated you yet."), and (b) the FSF is under the impression that strong copyleft exists, which is practically not true (there have always been Purity Hoops that you can jump through to make the two interoperate) and may be legally not true (Ubuntu's stand in including ZFS is precisely that all copyleft is weak copyleft, and the only difference is if your walls are implicit and poorly defined as in the GPL or explicit and well defined as in the MPL and CDDL). So it's in their ideological interests to consider things as incompatible even if they grant the Core Software Freedoms plus the Copyleft they like so much. Like, license incompatibility is the core selling point of GPL. The actual legalese of the license is really rather confusing and long by comparison.