Remember that "certus" is normally an adjective. If you decide to take it substantively, you have to decide whether that extension of meaning is justified. I can't say whether it would be incorrect for the author to have used it this way here, but it feels awkward to me. I would expect something like "nÅn certÅ sciunt" if that was the sense intended here.
Likewise, the sense of "with fruits of wisdom..." I would expect to be expressed in Latin as "fructÅ«s sapentiae habentÄ“s" (direct object of a participle) or "quibus fructÅ«s sapentiae sunt" (relative clause, dative of possession). I can't think of a type of dative or ablative which would have this sense. Ablative of accompaniment uses "cum", and I think is only used for people. An ablative of description would make sense only if it were "sapentiÄ" but not "fructibus", since "fructibus" can't describe the quality of the old men.
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)