I've received a huge amount of advice/insight from the experts on PrepLounge that have shaped my decisions on which companies to apply for, but not sure if it is deceptive to mention them in interviews/cover letters as we haven't actually "met" them. What are your thoughts on this?
Is it okay to mention insight from experts from PrepLounge in your interview/cover letter?
I assume you mean specific insights on firms - e.g. "Bain's focus on Private Equity, especially in London, is very attractive for me". In this case, there is nothing wrong with giving these are reasons - if anything, I would encourage you to use these insights as they (hopefully) should be non-generic and can show that you really did your research and thought about the question "Why Consultancy X?"
However, I would advise you not to say "from an expert on preplounge". Rather, say that you spoke to some "current/former employees".
Hope that helps!