What limit are you actually talking about? The 'limit' defined in VIII-1 U-1(d) is for pressure design (conditions), which is something else than the hydrotest, which is a test condition.
If you read U-1(d), you'll notice the 20 MPa (3000 psi) is not a 'hard' limit, but a limit where the Code addresses that the common DBF rules are usually not well defined for these conditions, and extra provisions are required.