"It makes me wonder though, if instead of looking at entrepreneurship from the software development side of things, maybe instead I should be trying to build a marketing and advertising agency that builds software."

Many people do this already and it's a highly competitive space. Stories like this are neat because it seems like a lot of money (and it is for a 16 year old), but many of these things are too small to fit into a marketing agency model to make them worthwhile.