I don't believe this is true. I think Paper's Please is the best recent example -- we've had the technology to make this game for decades, and it's never been done until now. I suspect there are quite a few concepts out there that are able to be realized by solo developers that just haven't been thought of yet.
In regard to the author, the type of games he made are a dime a dozen. At this point they're unoriginal and uninspiring, and it shouldn't be a surprise that they failed to catch on. I think marketing is extremely important, especially in the app market, but you need to have a product that you can market. This is the biggest problem.
edit: Also, naming. Naming is extremely important. Why would you name your game Letters? Are you trying to make it hard to find on search engines? For anyone making an app or game out there, spend a lot of time on picking a name. Make sure you're the first result in google if someone types in "your_game_name_here game". "Letters game" returns 208,000,000 results on Google, it's not feasible for someone to find it that way, which just hinders word of mouth. It's also hardly rememberable.