Aero and mountain bike don't seem to get along comfortably. Maybe the Shimano engineer was a Jetson's fan.
Aero bottles, like everything aero, matters ... after you've aero'd everything else possible. It might make slightly more difference than shaving our legs. For a shorter time trial the most aero bottle might be none at all, at least on an aero frame.
Biggest improvement in my average speed came from getting lower more consistently on rides, especially on windy days. But it took a lot of physical therapy to be able to get to that point. If I'm recalling correctly from some published wind tunnel testing results, the next most significant aero factors come from snug fitting apparel and an aero helmet. I think shoe covers were significant too.
Kinda makes me wonder why those fat, hollow tube carbon frame bikes don't include an option to use the downtube as a water reservoir, complete with aero straw running through the tubes and headset.
But with cable-free electronic shifting now, maybe the next big thing is to teleport water from the downtube reservoir, no straw needed.