I spent about an hour last night using forum and google keyword searches to find the post that linked the character, but it's been quite a while and I wasn't able to. But I'm almost certain it was a 19 ret paladin with an ilvl 56 epic neck. Unsure if that ilvl was before or after patch hit for ilvl squish. If I find it I'll link it here.
As someone else mentioned, it might take more-so around 11 accounts to do so.
The idea is that you use one account to tag an add (start at ilvl 89 I believe?; legion mobs scale down to 98), then tag it with a character that's 9 lvls below that, then another that's 9 lvls below that, and so on, until you reach the toon you want it on. Supposedly it works like a chain, in that you activate some way of surpassing the level gating required to loot high-end mobs (which is like 10 lvls). So in theory, you do this over and over (words can't express how grueling this grind would probably be) until you get an epic drop. Hopefully for your lowest character in the (raid) group.
There may be more to it, but this is the idea that's been shared around the forums at the time we noticed low lvl accounts with legion boes.
They may or may-not have already patched this. We just haven't had the willpower and/or money for a small group of people (or 1 person) to use so many accounts and try to pull this off to test.
Greens supposedly do not scale down such as the epics do (minimum scaling is 98). So it may only be the epics that behave this way, making the farm more grueling than one would think since it's constantly making you wonder if the whole trick is even workable.
I tried this farm once for my 91 twink at the time, and probably spent about 12 hours before throwing in the towel. Not about to spend my savings on on 10 more accounts and over-clock my computer for a small chance at something that people would simply report.
Who knows, maybe the shitface who got it tickled a GM's taint for it.
Still farming them boxes. 
