Sometimes USB keyboards with PS/2 backward compatibility have 2 plugs, one for they keyboard you can use with a USB to PS/2 passive adapter, and one that supports a built-in hub or some other secondary high-power feature such as RGB.

I suspect this has more to do with PS/2 compatibility than it does requiring a second port's worth of power.

My understanding was the second plug powered the LED lights in the keyboard but now I'm not 100% sure if its for that or just for the passthrough USB port on the keyboard itself.. maybe both?

Not OP, but presumably the keyboard is fitted with a USB hub requiring extra power.