That's some excellent feedback! Many thanks to all who responded with explanations.
So, it can be considered a settled matter that a 'glaive' should look like a polearm with a sword-like blade at the end for the purposes of WYSIWYG. And this leads us to the point already mentioned by Bca11:
I do find it annoying when GW doesn't issue a model with mandatory equipment, or no model at all (anyone remember 3rd edition wave serpents?).
In fact, it would seem that in pretty much all other instances GW has updated all wargear options to specifically match the options available in the corresponding sets. That's why, for example, Autarch with wings has no options other than fusion pistol and power sword - those are the only weapons available on a standard model! In fact, they seem to have been meticulous enough to even remove the Forceshield from the wargear list of an Autarch Skyrunner - indeed, unlike all other autarchs, jetbike autarch doesn't have the shield sculpted on his left arm.
Now, we also have a codex entry of an autarch on foot armed with a melee weapon referred to as 'star glaive' and not even having a shuriken pistol. Seeing how meticulous the GW designers were with all other HQ entries, I think it would be unreasonable to suggest that in this particular instance they suddenly decided to design a (pretty weirdly equipped) character that has no model to support it at all. Rather, it seems that this entry does relate very closely to one specific standard model - that of Prince Yriel
Indeed, technically Prince Yriel is the one and only standard model of an Autarch on foot that is still in official production. Also, he's armed with a very glaive-looking polearm, and has no sign of a pistol holster anywhere on him. Coincidence? I don't think so. In fact, now that I saw some great explanations and illustrations on the subject of glaives, I'm 99.9% sure that the 'Autarch' entry in our current codex is but a designers' cunning trick meant to enable the new Eldar players to buy the model of Prince Yriel and use it without any conversions to represent a generic Autarch on foot in any Craftworld.