I think you're confusing 'pencils down' with 'no further design work to be done'. When I heard that phrase, my assumption was that the design v1 would be done - and then ready for review, changes as required, updated, rinse, repeat. After the final design, there are validation activities to be completed, real testing, stack up analyses in prep for creating drawings for mfg parts, molds to be made, more tests, etc.
What in the world is going to take another year? Well, the X mfg process is still being refined after first ones shipped just under a year ago. Maybe they want to avoid that with the M3 launch. They can't afford small issues with a large number of products shipped.