Who owns the common property and can decide how it should be used? You and the other 11 owners or the strata manager?
What motivated the strata manager to issue the edict? Were they simply repeating an owner’s complaint or acting on their own initiative?
I would suggest that with only this small area of common property available for a BBQ, seating and some garden beds, the best solution would be to talk to you neighbours about formalising how you would like to use the area. You could collectively draw up a simple diagram showing garden beds, some paved area, some seating (with storage under), a space for a BBQ (noting that there is currently one on loan from unit X) and whatever else people would like. Perhaps a picnic table next to the BBQ rather than a storage seat? When there is a consensus or at least a substantial majority in favour, put a resolution to a general meeting to say that diagram shows how the owners corporation wants to use the space. That should put the manager (or one grumpy owner) back in their box.