It sounded farfetched to me, too, when I first heard it, which is why I bought that book. It doesn't cover it in too much detail, but I did find these two references:

Preface, page ix:

"Citrus does not come true from seed. If you plant an orange seed, a grapefruit might spring up. If you plant a seed of that grapefruit, you might get a bitter lemon."

Page 35:

"Up from those lime seeds came sweet orange trees, tangerines, limequats, citrons..."

OK, a little work with google once I got the term-of-art, "come true from seed", seems to reveal that not all species of citrus can do this, but that some of them can.