It is best to raise the tree if at all possible. Just exposing the root flare does not allow any fine feeder roots to have necessary oxygen exchange and access to moisture and nutrients that typically occur when they are in proper position right below the soil surface.

Raising the tree involves digging it out and redoing the planting hole so that the rootball rests on a secure (not sinkable) base that puts the root flare just above the soil surface and the feeder roots just at or slightly below the soil surface. And considering the lack of planting knowledge the nursery displayed, I'd also want to ensure that all wrappings were properly removed or loosened and that the roots were spread correctly and any circling or girdling roots corrected.

Not the best time of year to do this but better now than waiting and potentially permanently compromising the health of the tree.