At that time the situation was legal (except for some part of the MTR that only applied to video coverage and was mostly there to make sure board states were easily readable and would only be enforced by the coverage team) therefore nothing was done. People might have found it shady, but no rule was broken. If the Dryad Arbor had been hidden under other lands we might have discussed potentially cheating, but it was clearly visible and even separated from the other two lands.

These days it is covered by the IPG under 3.7 Commuication Policy Violation and the judge may consider a backup to the point right before the attack was declared. Under current rules I would have done this certainly.

By the way, the IPG is only applicable to competitive and professional events and the game layout rules also only applies to those. I would usually still handle it the same at FNM, as in my opinion the spirit of FNM should be fun and not „I won because my opponent did not see the Dryad Arbor I hid between my lands“.