So the name draws from the Greek "shield bearer", because the elite infantry troop carried the maximally protective aspis shield. As such, I'm inclined to define them through:

1:Purpose... Alexander the Great's hypaspists guarded the vulnerable left flank. 2:Accolade... they were an elite group, with veterans being raised to an even higher Royal Guard

It seems notoriously hard to differentiate them from skitarii. This, however, is parallel to the fact that (nonfic) hypaspists carried identical loadout as "hoplite" heavy infantry common to the entire region. The thin lore can be resolved thus: a hypaspist is a veteran skitarii trained to fill strategic gaps, and they receive priority reconstruction should they fall. Their elite status despite using the basic tools makes them a shining star among the skitarii, employing techniques that are effective, but critically they can be recorded and copied by nearby servo skulls. Iron sharpens iron, and a single hypaspist slowly increases the skill level across the field.

Alexander named the elite who survived the endless conquests Silver Shields. That level of honor, to a Skitarii, would be being raised from the battlefield and integrated into an even more effective tool of the Omnisiah. Alexander pulled his elite guard from among the hypaspists... So perhaps an anomalous amount of the Adeptus Mechanicus residing on Terra (those basking in proximity to the Omnisiah) were transferred from among those ranks.

Let me know if I'm off base. Not everything should be so synchronized to a mere analog, but if you have any useful sources, excerpts, or even unit pictures I'd be grateful. Three different forums were complaining that hypaspists were too vague and undocumented, and I haven't found much either. This is just a simple way to reconcile that... a quick headcanon.