They're not directly selling to teens, but they are indirectly selling them... and the entire big tobacco industry has been doing so for decades. You get them hooked early in life so you have a lifetime customer... that's been their tactic
This article mentions that he accidentally discovered this in 2009... here's an article from 2006 mentioning fragging https://www.advancedaquarist.com/2006/3/aafeature3 that also mentions that folks were doing it in the 90's
It sounds like you can fragment a larger coral, put them close together, and as they grow they'll actually fuse together, instead of competing, and becoming sexually mature much sooner.
Typically speaking, and based off a very small amount of experience of my own keeping a nano coral tank years ago, coral of the same type (zoanthids of different colors in my case) would run into each other, compete for the same space, and sometimes kill or drive back the other.
This is something the reef tank community has been using for years, in fact there are whole businesses built around the practice of fragging and selling corals... to the point that there is already equipment made for this purpose.
From "frag tanks" built wide, long, and shallow to allow light to have an easier time reaching the corals, to "frag plugs" made from ceramics that allow people to glue coral frags to to slot into existing rock, egg crate, etc.
I think maybe the accidental discovery was that the researcher found that this very common procedure to cultivating aquarium corals ended up also promoting the growth speed too.