Pretty sure that Planet means that support for the other OS's is planned, but not available right when Cosmo's are first released. They are giving themselves wiggle room for when the first Cosmo's get shipped.
Hopefully, it won't be too long of a wait......as I use Debian on my Gemini and would like to do the same on my Cosmo.
If you have a look on the gemain github site you will see there has been some commits for the cosmo linux kernel. I expect that it wont be to long before linux will be available. I have no knowledge of when linux will be available and I am not a developer so I can only go by the commits I have seen. We some very smart developers that might have access to the hardware and I am sure they want linux working as soon as possible.