You can use an 11 speed chain probably all the way down to 8 speed w/o having that big of a problem. I have used 10 speed chain all the way down to 7 speed, lower so would not surprise me that the 11 speed chain could work fine at least all the way down to 8 speed...
Straight blocks are rare now a days unless you make your own like miche does, I got a miche 11 custom made 12/13/14/15/16/17/18/19/21/23/25. Back in the day were more common for the guys to use those straight blocks, personally I like them a lot because sucked to have a 2 teeth jump in which one was going to be way too much gear and the other was going to get you dropped because the guys were cranking it really hard, specially with wind. Many times we were hoping the day we were able to get half teeth cogs but those ones were not going to happen ever
Racing is a strange world... is not ruled for what the 99% of the market buys and have in their bikes. Thats the reason of those straight blocks.
BTw they put the 11 speed chain detail in there just to be able to move them out, is not that the thing has 11 speed spacing, those are 9 speed spacing and maybe the teeth are a little bit thinner to save weight because the gibly was a TT line of wheels, even the gibhly stuff used a spacer in the back I believe aswell.
hope this helps.