@McTag,
This is saying that in this case, knowledge is a weapon. Again the key is to understand that the word "arm" is "to provide with a weapon".
The word knowledge is irrelevant, in fact this usage could use many different nouns.
Armed with a large stack of chips, he is ready to move on to the next poker table.
Armed with Windows 10, he will be more productive.
Armed with Ajax (I've a feeling of cleaning power).
Armed with a fanatical devotion to the pope ....
Graffiti artists are armed with a spray can, and writers are armed with a pen....
You can really be armed with anything that can be seen as a metaphorical weapon. "Knowledge" is only one of those things.