That’s not what it says, and to be honest, if it is/was, it’s wrong.
If I had a 3 ft , 6” diameter body fire eel living its life out in a 120 there would be outcry, and rightly so.
Most of the questions I see asked about tank size are either someone dreaming of what they may someday do, or are someone trying to justify what they are already doing.
If I catch a 3ft wild fire eel I might put it in 50g tank to photo and document, doesn’t mean the minimum tank size should be 50g.
I applaud people for wanting to give a fish a good honest home, however the question needs to be asked correctly.
Are you planning on buying a full grown fire eel and keeping that in a tank - if so my answer is VERY different to you planning to buy 4” baby fire eel to grow to adulthood.
If your doing the obvious sensible thing then I would expect the fish, and you, to have numerous changes of home by the time it gets to over 3 ft and on to full adult size and old age.
I would expect a 120 to be a good size to keep a baby fire eel for a number of years. Is it a tank for life, no, but I would expect every owner to “manage” that over time.