Slight inaccuracy, the data only goes back to 1979 and has not yet been verified by NOAA which has data going back to 1880.
It’s also worth noting that this is based on the Climate Reanalyzer which is intended for forecasting temperatures, not record keeping.
It would be more accurate to say it was the hottest day ever recorded by the Climate Reanalyzer.
Source: https://time.com/6292103/worlds-hottest-day-preliminary-record/