Even if we go back to strangers, the setting sun still marks dusk. What does it mean?

It means: Although we say goodbye and we will be strangers in the future, the setting sun will still leave its mark on the dusk.

It can also be understood that although we are now separated and we are strangers, we can still think of dusk when we look at the setting sun, and there is still a mark in our hearts when we continue to cut and sort things out. It describes two people, even if they are separated and no longer together, they still have good or bad marks somewhere in their hearts.

The original poem is: The sunrise in the east wakes people up, but it cannot understand my heart as well as the sunset. Even if we go back to strangers, the setting sun will still mark the dusk