In TFS 1.2 this is probably not controlled by walkable.
For this case, check the monster flags:
Orc Leader usually has:
<flag pushable="0" />
<flag canpushitems="1" />
<flag canpushcreatures="1" />
And Wild Warrior usually has:
<flag pushable="1" />
<flag canpushcreatures="0" />
So the Orc Leader is not pushable, but he can push other creatures. Since the Wild Warrior is pushable, the Orc Leader can move into its tile and push it, which can look like stacking.
If you only want to fix this case, try changing Orc Leader to:
<flag canpushcreatures="0" />
Or, if you do not want Wild Warrior to be pushed by other monsters, change Wild Warrior to:
<flag pushable="0" />
walkable="0" will not solve this because the stacking/pushing logic is handled by canpushcreatures and pushable, not by walkable.
If you want to disable monster stacking globally, then it needs a C++ change in Tile::queryAdd / monster push logic, but for this specific example the XML flags should be enough.