Not underground at all - literally the #1 rated tour in Orlando on TripAdvisor is a Disney VIP Tour business. There’s absolutely no mystery as to how they work either - nobody quoted for a national article is going to say “yeah, there’s this thing called DAS and you can just get return times and bring your group back to walk onto the rides.”
The problem they have is that they tell guests to leave five star reviews around the internet to drive business - almost all of which have names and some of which just say the quiet part out loud. Read this review and tell me whether it’s legit or not:
A must! Believe the hype, it is the only way to experience the parks