Optimization levels aren't going to care if you're using std::accumulate or not.
They'll either analyze the loop directly, or they'll analyze it after munging it through the template function instantiation code and inlining the results of the template function.
After that, both are doing what you originally said, lifting the C into an abstract form and analyzing it for whether parallel computation can be applied.
Hell, the code to SSE enabled asm examples they give in the comment you link are just plain for loops.
They'll either analyze the loop directly, or they'll analyze it after munging it through the template function instantiation code and inlining the results of the template function.
After that, both are doing what you originally said, lifting the C into an abstract form and analyzing it for whether parallel computation can be applied.
Hell, the code to SSE enabled asm examples they give in the comment you link are just plain for loops.