Since a trial halt at 75% would need p=.005 or possibly p=.001 that 42 days means that with the few extra patients at 42 days the DSMC thought that was a good possibility. If they didn't think it was achievable they wouldn't ask for an interim.

That they didn't increase the number of patients for the full trial or add 42 days means that they thought that p=.05 or less was easily achievable with current protocol. If they thought it could go either way they would err on the side of caution and ask for a protocol change.