Well that part of not coming true from seed is partly true. It depends on specific variety. Most often the more nucellar types come true from seed. But the hybrids like tangelos and other hybrids usually lean more on producing offsprings that are not true from seed. There is a listing here to show how general varieties behave. There are literature thatl classify tangelos under tangerines, even marketed as a type of tangerine, and so the statement will be somehow justified, but doesn't cover all generalizations that all tangerines do not come true from seeds. But saying that a grapefruit could come from a tangerine, that would be perhaps a couple million years of mutation.
looks like a grapefruit but tastes like a tangerine.. mmmmm.. 