They are not proposing a 1:1 relationship between use in games and medics being killed. That's not really how the meaning of symbols change. They are saying that using the medical symbol for game or media elements who are appropriate targets dilutes the clarity of the symbol. I think the argument is pretty straightforward and I'm not sure how one would "document" a symbolic dilution beyond suggesting that it will happen.
Also, if you are asking for examples in media, they are extremely easy to find. Here's an example from a comic strip commenting on TF2: https://photos.smugmug.com/photos/217534009_hZ5oD/0/1050x100...
