The magic threshold is the magic bonus the NPC is expected to already have. The level 11 guy already has a +2 bonus to defenses and such (details are in the text, check it out); the level 26 guy has +5. When you add a magic item, you only add the part of the bonus that's over the threshold...
Since the DMG calls statting out NPCs like PCs "too much work for what you really need", but says nothing about changing the XP value when you do, I assume they're supposed to be worth the same as a normal monster. How appropriate that is, I haven't the foggiest.
Right! :D So to make my statement less ambiguous, one might also argue that "hitting multiple targets" is largely synonymous with "physically injuring several people". (My point in any case is that you can push people through it. If that places it in a category other than "obstacle" and...
... but since you can't enter an obstacle (as per page 284), but you can enter the wall of fire, we know for a fact that it's not an obstacle. One might also argue that "hitting multiple targets" is largely synonymous with "attacking several people".