Same goes for Ipads and Iphones, they just don't have the hardware to run the game, and if it somehow did, it would more likely burn it out, due to the heat it would generate, trying to run the graphics.
With this not optimized engine for sure. But also, you know what spectacular games can be made with Metal on iPads or OpenGL/etc on Android. There are games with much more complex animations and engines than this.
This engine is far from optimized or usable in anything else than a PC.
It's good for games like this, probably free to use, so it's OK here.
Unfortunately not usable on phones/tables (for now). In 2-3 years for sure, the hardware in some phone/tablets already surpasses some low-mid level laptops.
Imagine this running on an iPad Pro, with an optimized engine.
Kst is great at using this engine, he did a great job with it, compared to other games. There's still room for improvement, but he can't do anything about the performance the engine offers.