Disney has a big construction project currently underway building road fly-overs and rerouting existing traffic patterns to make what you asked about very difficult.
Their goal is to force anyone parking in the MK lot to have to come through the MK Toll booth and pay (now in your case since you are staying on property you wouldn't pay but you would still have to go through the toll booth). They are pushing a lot of dirt around and pouring a lot of pavement to make this happen. There is an exit off World Drive as you approach the Toll Booth that allows people to go to the MK Resorts (GF, Poly, Contemp) but there is no access to the MK Lot via this route. If you are on it and intend to park at the MK lot, you have to head for the exits out past the gas station and car care center and they have a return lane on the left side for you to make a 180-degree turn and head back to the Toll Booth. The old return lane on the left at the gas station that used to take you to the MK lot is no more.
I guess so many people were avoiding the parking fee (locals coming in the back way, folks driving in past the Fort entrance) that they decided to make this change. So the answer is you still can drive to the MK lot and not pay as an on-site guests but you have to go well out of your way to do so which I guess is what you saw.
Do you have a concern about using the internal bus system to get to MK? You can catch a bus to WL from the Settlement Depot at the Fort down by Trails End and transfer at WL to their MK bus. Or use the boat of course.
Bama Ed
PS - we bus/boat to MK and drive to the other three parks.