Yes, you are misunderstanding the term. Whether a game is linear or not doesn't depend on whether there are invisible walls or not. Linearity refers to the storyline, not... "potential walkline". None of Bethesda's games have multiple storyline endings Morrowind included, unless you count your character dying and you not wanting to load a save to continue playing but then basically all games have multiple endings. It doesn't matter that you can do various non-main quests in various order and choose your own pacing of going through the main quest (you can basically do that in BioWare's games as well). What matters is how forked the main storyline is and how many different endings it has and how your choices affect it.
edit: For your information, whether the game has extensive invisible walls or not determines whether it is "open-world" or not, not whether it's linear or not.
For some linear is gameplay.
And D2 has a linear story too, so fail.
Unless you think having the same ending, but swapping 2 words makes it different ? DA2 is linear, in gameplay and story.