Prestige Classes often do not add a Weapon and Armour Proficiency entry if they grant no changes, so the [PrC] Diabolist lacking one is not necessarily a mistake. Looking at A-C alone I found the following without the entry:
Note that the original Volume 2 specifically stated that it did not grant the Diabolist any Armour/Weapon Proficiencies, so the entry may not have been included in the updated/collated version due to irrelevancy.
The Diabolist appears to lean more towards the stereotypically non-Melee, d6 HD Arcane Spellcaster than the others, using other Creatures to perform in Combat and gaining Spellcasting/(Sp)/research bonuses. Of the other two PrC, the Demoniac seems to be more Melee with d8 HD and augmentations to their body, whilst the Souldrinker is more close-Combat Spellcasting/Immunities. In addition, the Diabolist gains the Hellish Soul (Ex) Feature, enabling them to be restored to life normally, which the others do not get an equivalent of - possibly in exchange for the loss of Armour/Weapon Proficiency?