That’s been IX’s unofficial release date for a while now. I don’t know how that first became known, but in any event it’s official now.
The more interesting news here is that they pushed Indy 5 back a year (it was originally summer of 2019 IIRC). July 10 feels like such a random date, though. Maybe Paramount’s already staked out a Transformers movie for the end of June that year?