I think you are right. It seems to be the average time it takes to walk to the attraction after actually getting into the park (after the tapstyles and security).
The walk times & corresponding distances between attractions seem to bear that out. For instance, with a 9 am start at MK, my TP has me arriving at JC at 9:10 am. Later on, the time from Splash to HM is a little shorter, at 8 minutes, but the walk is proportionally shorter, too.
However, I think that if you did rope drop and were near the front, it would be less time than what is shown on the touring plan- or at least you’d get to your first attraction earlier than the plan says.