So wait, did he make all the models or was it taken from other sources? Seems to admit he took assets from elsewhere?
Because the impressive part would be if he made the models and animations from scratch, that is very hard to do, the underlying code in UE4 is still hard but plenty of people could do it considering the engine is much easier in this day and age.
Here is another issue with indie game development, it takes way too fucking long and you often don't make that much money from it. Very few are successful. It's easier to make a web service or website that makes you thousands a year, versus making an indie game.
Samuel Barnes
Ya he said on twitter that he bought the models and animations
William Cook
Then from a technical perspective this isn't as impressive, and the op's art design comment is not accurate.
If you had the assets, in Unity you could make this in a year starting from zero.