I can't believe how shallow the roots are. I'd read they are in top few feet of soil but seeing an actual picture of an uprooted tree makes it seem much more extreme.
I think some of the lack of much woody root system may have a lot to do with cultural practices like growing in poor sand, shallow irrigation and fertility management bordering on hydroponics, periodic high water table killing deep roots, and maybe the small containers most nursery citrus are grown in. 10 years ago most Florida Citrus were field grown and bare rooted, now all production has moved to enclosures & containers due to greening.