That IS mechanical damage. Nothing you can do but watch it encapsulate itself.

I feel like I would have noticed any sudden mechanical damage of that size.

If it was mechanical, it may have been smaller initially and grown so I'm now noticing it. If so, is doing nothing still the best course?

It could also be rot......from the tree planted a bit too deeply or having mulch piled against the trunk and rotting off the bark. Regardless of the cause - mechanical or rot - there is nothing you can or should do now except keep any mulch a distance away from the base of the tree.

Its odd. The wood looks like it has been exposed for a while, but the bark looks newly torn in some places yet not in others. The root flare looks good on one side, yet, due to the lean,may be deep on the other side. In the end, waiting is all you can or should do.

tj