When M is also zero, the value of h is not relevant. It cancels out. This part of the code is creating an achromatic colour with the same J value as the original, in order to run that backwards through the model to find the corresponding Y value and tone map that, because the tone mapping curve used operates in the linear luminance domain. The tone mapped value is then run forwards through the model, and the original M and h values “put back”. Ideally we would just apply a tone mapping curve to J, without the need for this back and forth. But we don’t have a version of the tone mapper that works in the ZCAM / Hellwig J domain.
The hellwig_ach Blink version here is a simplified version of the back and forth, which removes all the elements unnecessary for achromatic values. This is for Hellwig, but you could probably do something similar for ZCAM.