The "patent slavery" claim is therefore only credible if:
(1) - The GMO seeds in question have a local Kenyan patent.
(2) - Local Kenyan courts decide to enforce the rights of the patent holder, and toss arguments that "an act of God," or something similar, compelled farmers to use the GMO seeds.
I think that both steps are a priori unlikely. Virtually no patents -- even very important ones -- are filed in every possible jurisdiction. And I don't think that the courts would side with the patent holders in any case, for, e.g, force-majeure/act-of-god/unequal-agreement reasons.