I haven't had anyone specifically ask that I do NOT contact them through the AirBnB system (I agree that would be a red flag). However, I just had a guest who did not reply to me at all through the system. I finally had to call them to get the name of the other guest and their expected arrival time. After the phone call, I documented it through AirBnB's system by messaging, "Thanks for taking my call this afternoon. We look forward to hosting you and [guest's name]..."
By the way, my "day job" is running our web design firm, where I often deal with IT staff for various clients. I am often amazed at how little these people know. I have definitely learned that it doesn't automatically mean they know what they're doing just because they work in IT!