Probably for the best, as he is correct. Cover simply does not work like that, height(And even what the cover looks like) has exactly 0 bearing on cover. It's worth knowing that tracers from a hitscan weapon in CoH2 are merely visual, and them going over an obstacle or striking something between the firing unit and their target is meaningless.
I'm sure you've seen an MG set up behind a wall, with (almost) every round fired striking the wall instead of going towards their target? This doesn't actually affect the MG's damage or suppression at all, rounds that are destined to strike a model still phase through everything and hit him anyway, any rounds stopped were never going to hit anything.
Some weapons do have issues with height difference. I am pretty sure flamer can be effected by steep height difference.
I have not tested for HMG specifically but I would be surprised if there where issues on specific cases.