The diagram illustrates that W (which is common or white wire by code) is grounded at the main panel. Compare to the nomenclatural of the wiring for a 14-30 which is the more modern replacement for the 10-30.
Details in the link but the short version is
“NEMA 10s are classified as 125/250 V non-grounding (hot-hot-neutral), and were designed to be used in a manner that indirectly grounds the appliance frame to the neutral, which was common before the requirement of a separate safety ground was incorporated in the National Electrical Code.”