He did not use ench/alch/bs looping
Getting sneak high and getting dark brotherhood gauntlets isn't handwork or exploiting.
The game is stupidly easy for some select skills, without exploiting or working "hard" at all.
Is it too much to ask for a somewhat difficult play as an assassin? Do I really have to go out of my way to make sure I don't end up doing this?
What's funny is that I wonder what game it is you're playing.
Because I'm playing Skyrim on Adept difficulty, with my character at level 73, and numerous skills at 100: Enchanting, Smithing, Light Armor, Sneak, Lockpicking, Pickpocket, Speech, Alchemy, and I'm not getting these uber powerful weapons, and I'm playing pretty conventionally when it comes to whatever I craft.
I still get detected when I sneak. I still find enemies difficult and able to kill me, even with max armor rating, and I'm only one-shotting the low-level types.
But then again, maybe it's because I'm not playing my build to exactly one type of specification that emphasizes as much maximum damage output as possible? Kind of what an assassin is, especially when you're using the top-tier gear along with it, with all the requisite perks taken to further boost damage, you know?
Hell, I'm at level 96 on 2-Handed, using a warhammer with all the associated damage perks taken for it ( 5x Barbarian, 3x Skullcrusher) and with a very smithed glass warhammer, my damage tops out at 400. Unless I can solidly connect several power attacks in a row, only the low-level types are gonna fall in one hit.
Imagine that, making your character powerful is going to make him powerful.