HLG has a different look ("natural" to use BBC/BT.2408 terminology) versus the "traditional" look of SDR. To convert, you need to do a scene-referred conversion, which is not exposed in z.lib/vapoursynth. You can patch z.lib and build your own custom vapoursynth.dll, but really you're just better off using a LUT like shown.
The BBC has some official .cube files they provide for doing this sort of thing.
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