I'm a little confused (OK, more than a little). Why is he building a
wall, and would the soil be outside the wall, keeping the soil away
from the trunk, or inside the wall, where it would probably kill the
trees even more quickly? In general, if you change the soil level
around trees, you will harm the trees. The trees grow their roots
right where they want them to be, in the upper eight inches or so of
the soil for most of the feeder roots.
If you strip away four inches your remove half the roots, if you add
four inches of soil you smother half the roots. Maples have
particularly shallow roots. Some root exposure is normal on older
trees, given that we don't allow the leaf litter to accumulate in our
yards, unfortunately.

So, if the tree guy was suggesting that you build a small wall out
away from the trunk and then add a little soil outside of it to cover
some of the roots, that might make sense. If he was saying to build
it around the trunk and then fill that with soil to cover the base of
the trunk, crevices and all, he is likely mistaken. That would
increase the decay in the crevices and weaken the trunk. But I
haven't seen the trees, so maybe it is I who am mistaken. My advice
on "what to do and when" is nearly always the same regarding trees and
landscape: do nothing, most of the time.