Those 5k included miles are nationwide. Pick-up in Seattle, drop-off in Philadelphia.
Lets say I was in Denver, Colorado when I got the service message, so decided to drive to San Antonio, Texas, because Google told me that was the closest Sixt branch east of my location.

If I would have just done Denver to Philadelphia, that's 1700 miles.
But I had to do Denver to San Antonio to Philadelphia, which is 2700 miles.

Theoretically, Sixt would have to give me at least an extra 1000 included miles in that situation, right?