There is a lot of material online about NMR of varying quality, for better and worse. So I can understand your puzzlement.
It may help to remember that isotopes are simply variations of atoms of a particular chemical element with different numbers of neutrons. I think some people put across the idea that there is a primary variant (the one found most commonly in nature, usually) which is the "default" and that the rest are the isotopes, but I think that is a bit funny. The number of protons is what determines the element, after all, not neutrons. I don't discriminate - for example, all three isotopes of hydrogen are NMR-active. In the end, as long as you are dealing with a spin-1/2 or above nucleus whether it's naturally abundant or not, it will be NMR-active.