It's about historical equivocation around tge word "edition."
By the standard publishing term "edition," this is obviously a new edition since they will have new ISBNs, just like the mid-80's refresh of 1E was a new edition, and the black cover 2E books from the 90's were a new edition, and "3.5" was a new edition. These will be the 9th editions for both the PHB and DMG.
But TSR and later WotC adopted idiosyncratic, ad hoc uses under the moniker of "edition." 1E was not the original version of D&D. 2E started with the third typical edition of the rulebooks.3E wasn't the third of anything, being the 5th typical editionof the big rulebooks and like the 10th version of the game.