Bad advice :-)) Piling soil or mulch directly up against the trunk of a tree invites rot. And a thickness of soil over the root system can alter the oxygen exchange and smother the roots. The root flare of any tree should be exposed or no deeper than the current soil level. Even mulch should be kept at a sufficient distance from the trunk so that the root flare is exposed.

And mulch is all you need to conserve soil moisture. If you mound it up into a bit of a moat at the dripline, that will create a sort of well that when you do water, the water penetrates the surrounding soil deeply and slowly.

I am not sure of what he meant, but here is a few examples of how I would keep my trees...I posted a link and pay particular attention to 'Evans' trees in Arizona. His are the work of a pro and his grow so well his method..Look at the base of his trees and how he makes the ground catch water around them.

http://forums.gardenweb.com/discussions/4674573/hey-lets-see-your-june-2017-pics?n=72

Bad advice. Some people think that citrus should be planted with the root crown above the level of the soil; that is because historically citrus were planted on raised beds to let the roots breath a bit when they were flooded/irrigated in hot weather. No one does that today, as field trees are seldom flood irrigated; the practice today is more commonly drip irrigation or micro sprinklers.