Except that they neglected to stat out tanks in Rogue Trader. Were weapons in the basic sourcebook put there to take on stuff they neglected to put in the game?

I think not.

No, the truth is that Fantasy Flight just looked at W40K, and then made shit up without doing any math.

One of the basic principles of systems engineering is that general design principles must yield to individual system constraints. What do I mean by that? I mean that a tabletop wargame and an RPG have different requirements which override canonical considerations.

It's fine to have stupidly overpowered anti-tank weapons which are just as useful in an anti-personnel role in a tabletop game. Why? Because these have gear loadout restrictions, a short duration, and many interchangeable models.

This means you can't have everyone running around with meltaguns, and if any given model gets hosed by one, it's no big deal. Just part of the game. Something else will happen next game.

It's not fine in an RPG. Why? RPGs are continuous, and focus on the individual. When a probabilistic process is continuous, anything that can happen, eventually will happen. That means that if there is one weapon that outshines other weapons in most or all categories, anyone who can make acquisition rolls will eventually have one, and anyone who faces them in combat (because so many people have them), will eventually get hit by one. And "eventually" isn't all that long.

The problem isn't that the Inferno Pistol is too good at melting armour. It's that it's too good at everything else. It outshines other weapons in their own specializations. RPG designers call this "CAP", or "Cleric-Archer Phenomenon", referencing a design flaw in 3.5 edition D&D, where clerics could easy be built who were not just generally better than archer characters, but actually better at shooting arrows.

The RT weapon stats are rife with CAP. Many of those weapons have no reason to exist. And this violates a setting constraint, because the fluff tells us a lot about what the relative rarity of the weapons should be.

How I fixed melta weapons was not too weaken them (much), but make them less useful against things that aren't heavily armoured.

How?

Charge (X)
A weapon with the charge quality must build up power before firing, and imposes a small delay between the time the trigger mechanism is pressed, and the time it discharges. While this delay is not long enough to consume more actions, the wielder must keep the weapon trained on its target during this time, which makes hitting moving targets significantly more difficult. For each level of the Charge trait, a weapon imposes a -10 penalty to hit non-stationary targets. (Note that an enemy in combat who does not use a move action in that combat turn is not considered stationary, as such combatants are not truly motionless.) A charge weapon never gives a bonus to Ballistic Skill for firing at short or point blank range.

Recharge (X)
A recharge weapon must dissipate heat, cycle a mechanism, or build up charge after firing. After it is fired, it cannot be fired again for X combat turns.

I then assigned different melta weapons various levels of these traits.

Now melta weapons are just what you need against really heavy armour, but have drawbacks which make them less than ideal if you don't need that penetration.

Then I improved the armour ratings, especially for heavy armour, to increase the need for melta weapons against it.

Now, the inferno pistol goes from "why would you use anything else?" to a specialized weapon that excels in its defined role, and isn't ideal for other tasks.

Similar adjustments to other weapons produced a list where every entry has a reason to exist. Some are cheap and reliable, some are good generalists, some are specialized for this or that.