XGE gave us the option of having tool proficiencies (e.g., alchemist's supplies) enable a character to craft an item "as part of a long rest." For races that need 8 hours for a long rest but only 6 hours' actual sleep, that new way to use tool proficiencies made some sense. A character could be productive with his or her time spent at "light activity," i.e., resting but not specifically sleeping.
Since the August 2017 Sage Advice update, elves get all the benefits of a long rest in 4 hours, rather than 6 hours, if they trance during the rest.
How does an elf craft an item, per XGE, as part of his or her long rest?
I'm perfectly willing to hand-wave this at the table, but I'm curious as to whether anyone can come up with actual reasoning better than the unsatisfying reasoning I've come up with myself. (I'll keep mum on my thinking for now, lest I influence the discussion.)