I read the other thread as well. I believe they referring to barrels over 457mm. to chop it to under that is a no no. If you're already under that, chop away. I could be wrong, but what they're saying is that altering a barrel to under 457mm is bad. But what happens when the barrel is already under the 457?
Questar sums it up perfectly in this post. Regardless of the original length, if the end product is less than 457mm from cutting then it becomes prohib. You could cut a 20" down to 18" legally, for example. But if you cut a 20" or 14.5" down to 10.5", you have created a prohibited firearm. However, legally you can buy a new manufactured barrel/upper of any of those lengths and be okay. That's how I took it?
The grey area that everyone seems to be hinting at, is what is actually considered a manufactured barrel.