Most of the math in those files (and other includes I have looked at) is already at half precision, except where the comments say there is a reason for highp (or where it is really needed, like calculating SV_POSITION). It looks like it is even supposed to be using half for the interpolants, but somehow it mostly all comes out as highp in GLSL.
A lot of precision modifiers are clearly being ignored, for example:
UNIFORM_MEMBER_EX(float, DirectionalLightShadowing, EShaderPrecisionModifier::Half)
generates this in GLSL:
highp float PrecomputedLightingBuffer_DirectionalLightShadowing;