According to the Warlock class, the Pact of the Blade Pact Boon feature allows a Warlock to
use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it. You are proficient with it while you wield it.
Ordinarily, this would be very simple and straightforward. However, two-weapon fighting makes thing interesting. The Two-Weapon Fighting rule says:
When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand.
With the goal of using the two-weapon fighting rule above, let's use a shortsword as an example. Shortswords are martial melee weapons with the light and finesse properties. As such, they can be used for two-weapon fighting, and Warlocks are not proficient with them.
Now, perhaps I'm over-complicating things a bit... but it seems to me that there are two ways of reading that last sentence in the Pact of the Blade quote from above:
It could mean that because the Warlock has created a shortsword as their Pact weapon, that they are proficient with all shortswords while they have their pact weapon in hand. This would, in effect, temporarily add shortswords to their weapon proficiency list, and allow the Warlock to make a proficient shortsword bonus-action attack from two-weapon fighting.
It could mean that the warlock only has proficiency with their pact weapon; if they wanted to make a proficient bonus-action attack, they'd have to use a simple weapon in their off-hand.
Thus, my question is:
Does a Pact of the Blade Warlock have their weapon proficiency list increased while their pact weapon is active? In other words, is a Pact of the Blade Warlock proficient with all weapons of the same kind as their wielded pact weapon?