They said Tomb just finished up and is going to the printers now, which means there are still probably 2 months remaining on XGtE before lock.
If they decided to hold off on putting the Mystic and Artificer into the book, they now have some pages available. Perhaps they already have new rules in the book having to do with Tools (probably in conjunction with the new Downtime rules), and since they now have more pages available, they can expand some sections.
Thus, by Mike asking what parts of Tool proficiencies we want to see expanded upon, he can make a better decision on which parts to have expanded that we would actually want to see. After all... if every new section of this book has seen parts cut due to space... finding out which parts we'd prefer to add back in is not a bad idea at all.
And incidentally... in terms of what's been cut, it is my hope/guess that any rules having to do with the other campaign settings and planes were removed, and they will form the basis of the 2018 book. Once they decided they needed more time on the Mystic (Dark Sun primary rules) and Artificer (Eberron primary rules), they decided to hold off on any connection to a "multiplanar" book theme. So now I don't expect to see anything in XGtE about dragonmarks, or the three Dragonlance mage colleges, or kinder or any of that stuff. Maybe in 2018 once the two new big classes are finalized? At least, that's my hope.