Correct, it's very arcane knowledge.
Revenue manager of any, every airline will catch up to you but some of them are nice. Many are not. :)
Direct access to an GDS can also come way of an OTA, which, may not sanitize input properly, or if you learn how bulk purchasing of tickets happen (e.g. if airline is sold out, but it still shows/bookable by an OTA they have access to bulk ticket fares and can sell - sometimes flights close out on the airline but are bookable to last minute on OTA as well.)
Flyertalk is a great resource, and also, surprisngly the reason why Whatsapp was created too! - But I wasn't being indicitive of that and knowing what a fare code/y/j/whatever class fare is - I meant that sabre/amadeus and the front end of the airline are really "broken", and you can do similar things to what the original article posted, even abusive things like "walk" for tickets to refund to an airline hosted "Wallet" (usually you can attach a PNR and reuse it and rebook a new PNR/Ticket) which can break the chain and require someone at revenue management to really pull the log for that coupon/PNR to see where the money went, or why someone else flew that segment.
You can also double refund - and of course the airline can clawback - if they find out in time. Though many airlines have special gateway relationships w/ banks (as noted by the point system tied to a credit card network) (But this comes down as general fraud.)
The fuel dumping community isn't the only one, the reason they are unfriendly to newcomers is because they think they got a neat trick and they do, and once it hits front page news, that fare/date is dead. But there are other communities similar too - there is the staff travel/non revenue community which flies for free*, buddy passes, companion passes or on Zed90 tickets which is standby travel on OAL, other airlines (though you need to be listed/included on someones benefits.) and travel industry IATA or something - and thats the thing, if you can get access to a GDS and encode fares you may be able to list yourself but these aren't normally commercial fares, but I've seen many times a GDS spit out wrong class, class mismatch, fare/class mismatch, etc that wouldn't be a mistake fare per se, but a misconfiguration or a unsantized input that did this :)
But also, some GDS's are also nto that locked up, you can find some on shodan if you know the right keywords and literally poll someones name to see if they have any active flights, etc. I'm not encouring anyone to go find them, but if you know what to look for, there are many insecure or generally not behind any login page that opens up to a general query and allows reservations and confirmed commercial fares - but that falls down to just poor IT security operations in general vs GDS flaw.
tl;dr All Airlines front end/ecomm site talks to it's own GDS which are cloud hosted but act like 90's shell programs, some/many ecomm front ends are really bad and you can do stuff you shouldn't. It all comes down to your identity and is all tracable which is why many of these flaws still exist, because it's enforcable the other way/tech debt to fix is too much.
And we haven't even got into SSR codes, OSI codes, etc