I'm not sure, but I think they get that boost after a Shijin is called. At least the message about the monsters's strength increasing that pops up after spawning the Shijin says "monsters", though I'm not sure which ones they are referring to. Then again, the complaints about mobs hitting like trucks usually begin right after that.
Edit: The book on the very first floor also says that the effects of the Shijin affect all mobs in the area, so it might really affect everything.
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