Where else would the battery go without redesigning the entire vehicle frame?
It's not the battery placement - it's that the Accord (non-hybrid, and hybrid alike) isn't rigid enough (torsen) to allow for a folding rear-seatback. The standard Optima does get the 60/40 split. That this comes only with a pass-thru (if the picture is accurate) is suggestive that it's a cost-cutting measure.
I find a lot of Japanese cars do not have a folding rear seatback, especially the more performance oriented sedans. I think even the regular Accord does not fold.
My TL doesn't fold either, just a ski pass through. M37 and Lexus GS are same (at least the last gen, not sure about current gen).
German stuff always folds. Even my CLK with two giant doors and no B-pillar managed a
flat folding rear seat. I wonder how the AMG convertible version felt in terms of rigidity.