Mining is only one of many threats to Madidi.
I know one of these park rangers and, to my recollection, they don't ever carry guns in their line of work, though it's been a few years since we last saw each other and talked about this. The poachers and drug traffickers they run up against often do carry guns. Some of those participating are desperate locals, some aren't. Aside from the legal attacks, there have regularly been serious personal threats and even attacks on those trying to protect the park from mining and poaching.
These sorts of things have been going on down there for a long time and the people working to oppose them are overwhelmed.