Her original concerns with Marcos (before he ditched them) is the projections were not panning out for the belt to be self-sufficient without organics/etc coming from Earth, especially since Ganymede was still recovering from damage done during the Mars vs Earth stuff in S2/Jules Mao protomolecule story arc. This is consistent with the books - Marcos was envisioning a self-sufficient belt and grand "void cities" (think ISIS and their dreams of a nation), but the reality was the infrastructure and logistics were not there - they basically had stuff they could take by force and nothing else.
Marcos then basically steals shit from Ceres and peaces out, which magnifies an already shaky situation on Ceres. The acting head of Ceres didn't really have a workable plan at that point, she just knew what was missing. Even though Marcos fucked them, they still have no love for the inners. But they are in a fucked situation, so they really can't say no to whatever Earth+Mars do.
Earth and Mars don't have the ability to address the Ceres situation either, since they have their own problems and even if they tried to do it, it would take too long to transport stuff. The yeast is basically the unexpected solution to solve this (partially anyways)