Unity shaders are written in standard HLSL. There’s some basic boilerplate for Unity at the start, and a few Unity specific functions being used in the vertex shader, but the rest should be easily readable even if you’re only familiar with GLSL. The parts that use Unity specific functions would need to be written custom for any game engine anyway and cannot be written agnostically.

I’m also not sure why you’re surprised it’s written for Unity. It’s still the single most commonly used game engine on the planet.