The idea is that tilting the speaker to the rear moves the tweeter a bit further from your ears, so their output (in theory) arrives at your ears at the same time as the output from the other drivers in the speaker.because the tweeter isn't as deep as the rest of the drivers, and if they are all mounted on the front of the cabinet then the tweeter is actually a little closer than the other drivers.
All sound frequencies travel at the same speed. It is not true that
The tweeter cone has much less mass than the other drivers, so they propogate sound faster than mid-range or woofer drivers.