Has this been corrected in an errata that I just missed? Or should I just assume that if someone takes an Inferno pistol they're allowed to replace Pistol Weapons (Universal) with Pistol Weapons (Melta) ?

Perhaps you should read up on your talents...Universal Weapon Training (Melee) allows you to use Chain, Shock and Power Weapons and in my games also Primitive Weapons....the other Universal talents also grant multi weapon class training...

S.

Wouldn't it make more sense for the book to say "In order to use chain weapons, you must have the talent Melee Weapons (Universal)" then?

Yes and No,

You'll have to consider that RT is derived from Dark Heresy where all talents were sold seperately.
The only thing I didn't get was that Primitive Melee weapons weren't covered by Universal, so I house ruled it.

The Explorers are high level characters who are expected to know a lot of weapons since they have or at least had access to them, other characters, NPC and people such as hivers wouldn't and would have seperate talents.