Yes, if he tweets about an entire chapter then it's pretty much done.
And yes he quickly went from 413 to 418 but a year ago he was working on 421 (or so) and was tweeting pages.
I guess that working with batches of 10 chapter a year and releasing them when they're entirely done is different than week to week classic way of work.
Togashi has - due to his health and status as living mangaka legend - a very unique schedule.
He basically has the freedom to do anything he wants.
Whereas other mangaka are about 3 or 4 chapters ahead most of the time (only working on those), Togashi is currently simultaneously working on about 50 chapters, which he confirmed last December.
Those 50 chapters are all in various stages of completion.
The next 10 (411-420) are basically done and are those that he currently posts updates on. Only some minor background corrections have to be made.
The next 10 (421-430) are also mostly drawn. Here writing and inserting the dialogue is the next step (takes the longest), before the background adjustments happen as final step.
The next 30 (431-460) are in the storyboard stage (very rough sketches of the chapters).