I didn't understand that either. MongoDB works very well with many other languages; just recently I was using it with Python, and the driver and API work very well. I guess since MongoDB stores data in BSON, which is the binary version of JSON, the javascript object format, the author decided that JS is closest in spirit to Mongo.
The mongodb shell uses javascript, and stored functions and map/reduce functions and more all use javascript. So if you use mongodb, you'll be using quite a bit of javascript anyways.