The more I think about this, it is starting to make sense. I started this post in agreement / confusion with CapnZapp. Here's my current thinking; let me know where I might be misconfused.
Borrowing from the OP, with highlights my own:
New version (from this errata):
A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.
Original version (unchanged in Player's Handbook):
Trance. Elves don't need to sleep. Instead, they meditate deeply, remaining semiconscious, for 4 hours a day. (The Common word for such meditation is "trance.") While meditating, you can dream after a fashion; such dreams are actually mental exercises that have become reflexive through years of practice. After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.
New explanation from the updated Sage Advice Compendium:
Does the Trance trait allow an elf to finish a long rest in 4 hours? If an elf meditates during a long rest (as described in the Trance trait), the elf finishes the rest after only 4 hours. A meditating elf otherwise follows all the rules for a long rest; only the duration is changed. [This answer has been altered as a result of a tweak to the rules for a long rest, which appears in newer printings of the Player’s Handbook.]
So, if a human sleeps for 8 hours, he has fully met the requirements of a Long Rest.
Thus, if an elf's Trance grants this exact same benefit, then a 4-hour trance also fully meets the requirements of a Long Rest.
This COULD have been included in the new definition of a Long Rest "...except for elves..."; but I think they wanted to keep away from trying to list every exception in the general rules, and instead only put any exceptions to the general rule within a specific rule.
Unless I'm misunderstanding something, I think I am satisfied.
