It's not that it's so terribly hard to build an upper, but it requires specialized tools, torque wrenches, and clamping fixtures that I would have spent almost as much in tooling to build an upper as I would just buying a completed upper.
If I were into swapping handguards, gas blocks, barrels, or building multiple ARs, the investment into the specialized tooling is justified.
For my single AR, a stripped Poverty Pony lower, and a completed upper with a lower parts kit from PSA was the extent of my commitment to building an AR.
Going by the old adage that "two is one, and one is none," if the government ever asks, the number of ARs I own are 'none.'
All of my builds included doing the upper. It's the place where your choices in components can affect accuracy. The article does not mention lapping the upper receiver. Another tool, but one cannot have too many tools.