or it really is an inventory clearance event…
Sales of iMacs were likely slow for some time now given the old chip so the I guess the thinking is announce a new model, sales will increase and you can get rid of the remaining stock of lightning accessories.
Who knows what they've got planned for the mouse and keyboards. You would guess at some point in the next year they will want to move to USB C and maybe they will have a major revision of them to launch with an updated Mac Studio or something and these will then quietly come to the iMac.