To put things in perspective, this is a system with a 4MHz CPU and 8K of RAM, and has roughly the same compute power as an Arduino. Good example of how you don't need multi-GHz processors and tons of memory to do something interesting and entertaining.
What's the big deal? This has been on android for ages.
1) It's a port to a platform a lot of people remember fondly.
2) It's a port to a platform that is a little bit more arcane.
Z-80 ASM is arcane? Well, it is also has custom audio and graphics.