Eco mode does not use the starter/battery to restart. It's pretty cool. It fires the last cylinder that was closest to firing to restart. I googled it and it was very interesting.
It was a video from Mercedes I found that explained it from one of their engineers.How does it fire the last cylinder? I would double check your google search or post the link to your search. What you said sounds like what Mazda tried a few years ago which failed.