exaclty, the "any derivatives" clause. pretty much means i will not ever knowingly grow any OSSI seeds. i like what they are doing, i just cannot participate.
at my first seed swap a friend gave me a packet of the beefy resilient grex and then when i went and looked at the OSSI definitions i didn't plant them and returned the packet of seeds to her the next time i saw her.
i really need to avoid complications... i have plenty enough of those as it is.
if you want to also quibble about "totally free" you can do that too, since once you put a requirement on it then it is no longer totally free, but that is a philosophical quibble. one i'm familiar with from the free software community and various conversations over the years.