Hello and welcome!
I suggest that you put this question in the section "other WW1 aviation" and there you'll surely get some answers.
Concering your question: there were "a lot" of AA-guns in the area where Luke was killed.
He landed close to important German lines of communication and supply. The front was only a few dozen of kilometers away to the southwest in late September 1918. Additionally the hill "Côte Saint Germain" directly northwest of the village of Murvaux (Luke landed a few hundred meters southwest of this village) was an ideal place for AA-batteries.
Hope this helps a bit ...