No, this is not meant to be a time bar, and it does not act as one. Not meeting a Key Date is a breach of clause 30.3 of the contract. In that case the Contractor could try to recover damages in law for that breach rather than the more limited amounts set out in clause 25.3, i.e. “on the same project”. The last sentence of 25.3 ensures that the Contractor cannot do so, and that their only right is to use the calculation based upon cost incurred only “on the project”.
As a final point it is important to understand that clauses 30.3 and 25.3 only apply to those Key Dates that are listed in the Subcontract Data Part 1 of your contract. Key Dates cannot be added by the Contractor at will, they have to be in the contract.