From producer letter 16
The team’s working hell-for-leather to deliver this monster patch (did I mention that it’s a giant next to 1.18?) to you by the end of September. We’d love to be able to release the patch sooner, but all things considered, I’m afraid this is the [b]earliest[b] we can reasonably achieve.
To me this says were working our asses off here and were shooting for this goal but we might not hit it exactly. This is reinforced by the following statement earlier on in the letter

Just so you’re given due warning, the team is running on a tightrope to stay on schedule, so please don’t be surprised if a handful of things end up getting put off until 1.19a or later.
So, it will arrive when it arrives. Got to say though with the volume of info we have been getting as of late were probably getting close. And for all we know we could wake up tomorrow to find the preview patch notes on the stone.