It's incorrect though. Togashi in his last tweet basically says that his editors are still giving feedbacks and that he accordingly makes corrections
That just depends on what "complete" means here. But chapters 391 to 400 are certainly not all ready for release
You misunderstood sth.
The volume that is complete consists of chapters 381-390 which were already out since 2018 but haven't been released in volume form yet.
Togashi and his team put them in a volume now (lot of correcting background effects etc. and generally upping the quality compared to the weekly release).
Meanwhile he has completed the scripts and rough outlines of NEW chapters 391-400, but only 391-393 are fully complete/edited and ready to be shown in weekly shonen jump already. The rest of those still has some work/editing it has to go through.
Currently he is also working on the scripts and rough sketches for chapters 401-410, but those will still take a while until they're ready.