Hmmm seems to me that this is a social skill...using Bluff in place of knowledge skill? Not sure it matters that much but I would make the role play it...it feels like this is meant to be used a parties to impress people with the knowledge. I would rule that they can use it in social settings to impress people with the knowledge (be it fake knowledge) but it does not grant real knowledge just the appearance of real knowledge. At least that is the way it reads to me. Since it does not say you gain the skill it says you can use bluff in place of..so that means for every skill then want to use you can use the bluff rules not the rules of the skill they are replacing.
Seems rather harmless to me and really not that good