I think it depends on the target audience. If it's math grad students that have had a first course of DG and probability and statistics, then it's elementary. I think the statement that it's self contained is practically incorrect, it's hard to believe that someone that doesn't already understand manifolds and connections can get a good enough intro to tackle IG from the overview in those notes.

Addendum: What I mean is that you can get much more abstract and take many more things for granted than it's done here. It's like reading an elementary introduction to the geometry of schemes, even if it's elementary there is a bunch of stuff that has to be assumed as known, even if the explanation of schemes per se is elementary