I have no interest in defending this game, as like I said, I have close to no interest in the stealth genre and I only have a passing interest in this game because of its good graphics and good localization, it could be nice as entertainment when I'm horribly bored and have nothing else to do. But this feature in particular with the very customizable difficulty and HUD is actually well implemented. I suspected that he was pretty much talking out of his ass when he claimed that the changes are not supported by the gameplay, so I asked him as much, and he never replied. So I did some of my own research.
now in the HUD, you have the option to turn off the minimap, threat meter, threat icons and all this. Now all of this does make the game harder irrespective of the actual game design. It forces you to be more aware of your surroundings, memorize where you last saw your enemy and try to keep a mental overview, and guess where the guards are going. This is by definition harder, and does not actually require any change to game design to actually implement. The removal of the threat indicators forces you to look more at actual visual cues, which very much exists on enemies in thief, I have seen, to see whether they are aware of your or not. If I understand correctly what this option does.
Now as for the difficuly; chapter only saves - this is as well, irrespective of game design, an increase in difficulty. Having constraints on when you can save, does undoubtedly increase the difficulty for those who would save very often, and also introduces a level of psycological difficulty - the fear of failure, which ironically often makes you more prone to do mistakes, and fail. The fear of failure, and various psycological effects, is precisely where games like dark souls derive most of its difficulty. If you could quick save at any time in dark souls, not a single guy would think dark souls was difficult.
Most of the other stuff look like typical constraints that players would put on themselves to finish the game with more difficulty, admittedly the success of many of these depends on the design of the game. And maybe some of them does not work well with the design, but there are some that I can say straight away works from what I've seen and what little I know about the game, such as
-No speciality arrows
-zero damage
-iron man mode
-no alerts
-no kills or takedowns.
As the game is very much designed to be beatable never killing anyone, or indeed never being detected. Now this may not be true for all of the other options, I can't tell, but this already covers most of them. And even with a few such options, it's waaay better than not having any such options at all.