A basic question about direct access to the GPIO registers by the μC units
The boxes and arrows in these two diagrams have different meanings; it does not make sense to compare them.

TI's official MSP430G2553 web site shows a functional block diagram that is quite similar to the AVR one:

  

And the internals of the CPU also look similar:


(source: MSP430x2xx Family User's Guide)

Both microcontrollers work in the same way: the CPU and the GPIO module are connected to the address and data buses; when the CPU accesses a memory address in the correct range, the GPIO module acts on that. When writing to the appropriate register (PORTxn on the AVR, PxOUT on the MSP430), the associated port pin changes.