When the UART sends something out the Tx pin does change state, right ?
Of course, but I don't think this goes through the PORTD register, the USART peripheral controls the output driver directly, without writing to PORTD.
I'm referring to §13.3 of the ATmega328P datasheet, figure 13-5 specifically, as well as §13.3.3, table 13-11.
Enabling the USART enables PVOE (port value override enable) and connects PVOV (port value override value) to the TXD output of the USART peripheral. In other words, when the USART is enabled, the PORTD register is bypassed for the TX pin, and it is driven directly from the USART. As far as I can see, PORTD is not written to by the UART ISRs.