To me it’s about utilizing game mechanics that are known to not work correctly,
"Cancer Build" means "I am to the left of the intelligence bell curve." Same as using fanboi terms like "meta," "overperforming," "BIS," etc. Ever since WOW, there is a lingo that dumb people who take video games a bit too seriously use to try to sound knowledgeable... about video games that aren't particularly competitive. "Cancer build" is part of that "People of Walmart" mentality.