It just feels so out of left field for the administration. I'm not saying the Biden administration is perfect cause its not, but border walls are not something he has ever given any indication of having as a priority.

https://www.washingtontimes.com/news/2023/jun/30/joe-biden-forced-build-donald-trumps-border-wall/

Found this. Basically the claim here is that funds were allocated to this project, and although some of those funds were redirected, congress never deallocated the rest, and therefore they have to continue to be used. I'm not totally convinced but it's a more nuanced take than the comments here.