LiveAquaria has never been the most reliable way of defining a tank requirement for a fish haha. You are correct. I would honest tell you that those requirements should be flipped really.
Although Polypterus senegalus (cuvier's bichir, senegal bichir, etc) does not grow very large (comparatively to other bichirs that is) they will top out at about 10 inches (or at least that has been my experience...granted I never took a measuring tape to them). Combine this with the fact that the fish is predatory, will mean that the fish has a decently high bioload. This is why I would say a 50g tank is preferred. When you first get one, it most likely wont be any larger than 2-3 inches. It is perfectly acceptable to house this is a smaller tank, but just have plans to move it as it grows for the health of your bichir as well as any fish you choose to house with it.
Let me know if you need anything else as I have kept a variety of Polypterus