So a thing about steam and R18 games to remember. In order for a game to be released on steam as a R18 game. It needs to go through steams approval process. Steam is notorious for taking its sweet time to approve these games. Additionally, if steam finds a reason to deny your approval you will need to go back and fix whatever caused you to be denied, and then resubmit your request and wait again. So it is entirely possible this game is stuck in approval limbo. I have seen this cause games to be delayed for quite some time in the past due to steams' arbitrary bullshit policies regarding its games. Unless they have changed they way they do things I would wager they wanted to release on the 18th but got held up by steam.
Edit: Also reading a little of steams forums, the release date being set at the 18th might of been by error as it is again set back to just 2019.