Looks like you can some times move out and some times you cant? Its kinda hard to see at times.
The point where he couldn't walk out onto the grass in that one video was because he had an enemy in melee with him, which he didn't realize until he turned around. You can't walk out of melee in Isles of Terra/Xeen, so probably the same rule. The direction movement arrows were greyed out until he killed the enemy, but when he did the arrows started showing that he could walk onto the grass again. The guy just didn't.
Kinda interested to see how pathfinding/mountaineering will work, with the tile transitions the way they are you will basically have to obviously clip through trees. I hope they don't just let trees and hills be impassable walls and drop travel skills, being able to eventually go anywhere in M&M when you started off constrained was great.