Please, read the article with comprehension.
It’s NOT the Wash. Post, and no, it does NOT come from c1900.
“hang a Moro chieftain by the heels over an open grave, kill a pig, and then drop the Moro into the grave with the bloody animal,”
This was the letter written by 1 man 60 years later, NOT contemporaneously. (And yes, it would be amazing only 1 man witnessed it; why not more corroboration?)
It may imply but does not say the chief was buried, but he did have to mingle with dead bloody pig.
I've found two other sources for US Military in the Philippines using pigs to deter Muslim terrorists.
http://query.nytimes.com/gst/abstract.html?res=9F00E0DF1139E333A25751C2A9649D946297D6CF&legacy=true
http://historynewsnetwork.org/article/162096
And this as well.
http://heavy.com/news/2017/08/general-pershing-trump-bullets-dipped-in-pigs-blood-myth/