It's in the very name: "Tiny-GPU". Since it's a demonstration project by a hobbyist, the author probably didn't want to implement the whole optimized rendering stack yet.
On the other hand, they also left out some features that you'd expect to find on a general-purpose compute accelerator.
For example, they focus on tensor math. No support for bit wrangling and other integer math. No exotic floating point formats. Minimal branching capabilities.
Graphics pretty much boils down to matrix multiplication, and that's exactly what this thing accelerates. If it were a generalized accellerator, it would have to support other kinds of arithmetic as well.
