Agree that this looks odd. Not sure if it is intentional or a mistake. I tend to think it is the latter. SettlType(63) and LegSettlType(587) have the same datatype and enums (FIXimate says that 587 uses enums from 63). 975 has a different datatype (int instead of String), name (Settlement instead of Settl) and its own enums. Due to the difference in datatype, it cannot be linked to 63 with “uses enums from”.
There is a thread to capture errors (FIX Standard Errors and Omissions). I have made an entry and linked it back to this thread.