Interim Analyses of Clinical Trials, page-59

80% power is fairly standard for a trial. If they wanted to design a trial to detect a smaller GFR slope difference like 1ml/min and keeping the same alpha, then it would most likely drive up the same size needed, possibly by quite a lot.
As the earlier trials have shown nothing close to 30% proteinuria reduction with the DMX drug, I'm not sure why they think they will achieve that here but we'll find out at some point if the drug performs better than expected.
I recommend you read the DUPLEX paper published in New England Journal of Medicine because the authors offer a number of reasons why the trial missed its primary endpoint. A better than expected response with control arm of Irbesartan is just one possible explanation. They report imbalance in use of other drugs that improve kidneys, with more immunosuppressive drugs used in the control arm which could have affected the overall result too. As follows from the paper: "we speculate that eGFR slopes in the irbesartan group might have been differentially improved as a result of immunosuppressive treatments, potentially diluting differences between groups in the primary analysis."
Same thing could happen in DBX trial.