I'm sure you've observed all of that, because most of those captions are just someone creating their own narrative over footage of some really basic behaviors.
All of their examples of enemies distracting/flanking/surprising the player come from them backing out into the combat area. None of this would happen if the player just stayed near the door. The Fallen didn't sneak up from behind, the player simply turned their back to it.
Rushing players who are out of ammo? Boy, that would be useful when I ran out of ammo during the boss fight and couldn't get anything interested in storming my little hidey hole.
Certainly i haven't noticed any association between two events (such as out of ammo and enemy charging, my experience with charges is that they do them randomly, and unsuccessfully bullet-sponginess aside). And some things are atmospheric things, not examples of intelligence (like the AI taunting the player).
Most certainly none of those things makes the AI challenging to fight (it doesn't execute any tactics well enough really), its overwhelming numbers and endurance are much bigger challenges usually. It doesn't often win unless i make obvious mistakes (like the AI in Halo). Though the AI doesn't always exploit mistakes...
Intelligence-wise, not challenging enough for me. Strength-wise it isn't fortunately as frustrating (read: promoting long-range combat and slow, methodical approach as the optimal one) as the AI in Reach but it is still too much based on numbers and power.
It is interesting to note that low-level AI still has quite a lot of punch even if it doesn't have much health (compared to your attack power at least). A few very low-level Dregs can damage you quite a lot easily, if you basically let them. The intelligence level doesn't seem to vary with level either, lowbies and higher level ones don't exhibit differences in behavior really.
This is not a complaint, just an observation.
By the way, if you run out of ammo completely, you magically get some more.
That Devil Walker in the Blast (basically unused area outside the Strike but it contains the Spider Tank always)? I've killed it a couple of times with sheer patience. Shoot until out of ammo, stay in cover, repeat. Takes hellishly long but... And sometimes dying is a faster way to gain more ammo.