If the owners corporation had a landscape plan adopted as policy by a general meeting resolution that designated this area for growing weeds and if the committee changed its purpose to growing veggies without similar authority, then that might have been acting beyond its powers. If it spend a substantial amount on the establishment of the garden without a budget allowance, that also might have been beyond its powers.
However, now, 3 years on, the area IS growing veggies and I suggest the onus is on anyone objecting to put a motion to a general meeting to change the purpose of this area of common property to something else. Such a GM resolution would then trump the committee’s decision. The committee could put a counter-motion to establish a set of guidelines for the management of the area as a community garden.
Ultimately, the owners corporation as a whole can decide how it wants to use its common property.
BTW. When the place where I live set up a substantial community garden it involved non-trivial money and a change of purpose to an area of common property so we did put an authorising motion to a general meeting after considerable consultation with owners.