It is entirely possible that Diodorus has confused the technical terms here. For modern historians it is par for the course to write Diodorus off as a “clumsy copyist” who displays little or no intellectual rigour in that copying. Yet, it would seem, that he never confuses terms that Arrian – a stickler for terms and patronymics, etc – has a difficult enough time with.Taphoi wrote: It is impossible that he meant anything by this other than that Pausanias was an official and personal bodyguard of the king. He cannot have meant that Pausanias was an officer in the hypaspists...
Plutarch (Morallia, 170E) uses the term doryphoroi or “spear-carrier” – a term occasionally applied to hypaspists and which MM Markle (particularly) has pointed to in arguing for their normal armouring being that of the hoplite. Diodorus uses it himself at 94.3:
If we accept your absolute declaration that any other reading here is “impossible” then we must conclude that Leonnatus, Perdiccas and Attalus are members of the Seven – for Diodorus states it at 94.4:Every seat in the theatre was taken when Philip appeared wearing a white cloak, and by his express orders his bodyguard held away from him…
Odd that Alexander, years afterward, felt the need to promote Leonnatus and Perdiccas, the noted battalion commander, to the position - again.Immediately one group of the bodyguards hurried to the body of the king while the rest poured out in pursuit of the assassin; among these last were Leonnatus and Perdiccas and Attalus.
Perhaps it was like a renewing of wedding vows? Perhaps another, not quite so impossible, explanation is that Diodorus – as Arrian on occasion – has confused the technical terminology.