You’ve made better arguments than this in the past (see your own post cited below).
Salty is running circles around this thread because she can easily win a very simple logical argument against a flawed assertion.
She could very easily come out and say something like they ran 10 distinct runs of 10,000 simulated d20 dice rolls, checking to ensure that the result was an integer between 1 and 20. Not once did a simulated dice roll fall outside of these expected boundaries. Therefore, the RNG is not broken. Such a test would be trivially easy to run and would yield the expected results always.
But, that’s not what you’re really venting about in this thread.
What you are really trying to say here is that the RNG roll itself isn’t broken or flawed here, but rather that there’s there’s funny business going on with the result of that RNG roll.
In the form of a tabletop RPG anecdote,
Player: “I’m attacking the Treasure Gnome!”
DM: “Ok. I’ll make an attack roll.”
The d20 rolls a 16.
Player: Checks character sheet. “Great!, I only needed a 15 to hit!”
DM: “The attack misses.”
Player: “What?” /tableflip
The key piece of information missing in this anecdote is that the player was playing a Vault battle on Hard difficulty and consequentially their attack rolls had a -2 modifier to hit. This difficulty modifier was not informed to the player by the DM before the player started the encounter with the Treasure Gnome.
The problem here, IMO, is that certain game concepts, such as difficulty modifiers have become vaguer over time to players, while the devs (the DMs) are fully aware of these behind-the-scene adjustments to gameplay. After all, they have to know this information when testing and balancing content.
Once upon a time, such information was made available to players.
Spending way too much time digging in the forum history, I found a blast from the distant past on almost the same topic. Interestingly, many of the names in this thread also appear in this THREE year old post. 
Scroll down a bit in that thread to see a most interesting post by Ozball, citing Sirrian, describing internal AI scoring of “lucky events”. While this information is severely dated (and likely no longer accurate), that level of detail about the inner workings of the game would probably not be provided in today’s version of the game.
From two and a half years ago, I wrote this post on almost the exact same argument that @awryan is trying to make here.
Spoiler alert: We never got one. But, one by one over time, all of the game modes replaced their player-selected difficulties (with a half-exception for Explore mode where the player can select enemy levels), with the current system that does things in the background that the player can’t directly observe.
I also remember an ancient dev Q&A where Sirrian stated that the “luck factor” (which eventually morphed into the current Difficulty system) was added to battles to make them more interesting and more arcade-like. But, after over two hours on this post alone and its research, I think I want to go enjoy my Saturday evening and not spend another 2-3 hours re-watching ancient streams to find a very specific 30-second sound bite that really isn’t going to add anything more to this post.