The purchaser may well feel aggrieved but that doesn’t mean they have no liability. It’s up to the vendor to provide accurate information about future liabilities and the purchaser to check through strata records (as far as possible) that this is accurate.
The strata scheme has no responsibility for assumptions the purchaser makes, whether they are accurate or not. There must be some record of the failure to complete the work somewhere. It’s sad for the purchaser but this proves the value of a forensic search of strata records before you buy.
If the strata manager has told them that they don’t have to pay, that’s on the strata manager, not the committee.