Excactly, a dev stated that, and everyone thinks it is factual. To the contrary though, 8 man is easier than 16 man this tier. I have done both.
I have done both, but that doesn't matter at all to the community. The 8 man guilds that haven't done 16 man are obsessed with the fact
that 8 man is clearly superior and more difficult.
It's likely because a dev (pre-1.2) stated they wanted 8M to be slightly harder than 16M.
This is not possible. The only way the difficulty of controlling and spacing 8 members in an area would be even close to the difficulty of 16 man would be if 16 man was severely undertuned in comparison, and it's not. I don't want it to be, quite frankly I think most of the community would prefer if 16 man was significantly harder, with entirely different mechanics, and gave better loot.
They could cut out nightmare mode entirely and save a lot of development time by making 8 man hard mode and 16 man nightmare mode.