My goodness, as closely as the military is to the president, I feel very sure he was aware of this statute before he took office. In the meanwhile Pres Trump has put Dems on blast for the world to see that they do not represent the American people to the point of derelict if not treasonous agenda. Strategy has worked.

Democrats represent foreigners, not Americans.

Also per JAP's post here the wall will likely not be held up in court arguing eminent domain. 

Watch–Defense Official: Nothing Stopping Trump from Authorizing Military to Build Border Wall

"President Trump can build a border wall using the United States military to stop mass illegal immigration and drug trafficking without funding approval from Congress and without having to declare a national emergency...."

"....BROOKS:  I want to direct your attention to  10 United States Code § 284 which authorizes President Trump to deploy the United States military to the southern border to build fences and to do a lot of other things, and for clarity, if you look it up in the dictionary the word fence includes the word barrier and the word barrier includes walls made of a wide variety of different materials....."

https://www.breitbart.com/politics/2019/01/29/watch-defense-official-nothing-stopping-trump-from-authorizing-military-to-build-border-wall/