You are overlooking the "they didn't test it" part. Obsidian will have to test it, in all maps, entering cutscenes, in combat, etc etc. Testing is almost always takes the most time when you include something new. So I don't disagree putting it in game would be "easy" but Obsidian is not two guys doing it for a buddy, they have to do it right.
No, I don't overlook it, I personally just don't think it's that big of a deal. It's not like there are many different mechanics that intertwine with other aspects of the game - it's a simple on/off flag that changes one set of animation to another. Very similar to running and scouting, which is already in the game. To make it super simple: replace the scouting animation with walking - done. I literally "walked" through POE1 from beginning to end and did not encounter any problems. So if this feature didn't get tested before being implemented into IE mod, it either was coded perfectly in the first place or there is simply not that much behind the curtain that can cause problems...
But hey, that's just my opinion and you guys are more than welcome to disagree. I just don't think we'll end on the same page anytime soon.
It's not so much a matter of whether it causes problems or not. But rather by adding it means you have to check for problems even if there are none, because that's what good developers do.