Past perfect tense and Since

Lenny had slept most of the way since leaving Texas.

I found this sentence from Cambridge Dictionary. I have been taught that you use past perfect tense to emphasize an event in the past that took place before another past event, and you also need to use past simple on the event or action that took place afterward to form a complete sentence.

The quoted sentence seems to violate these rules. In this sentence, leaving Texas happened before had slept, and there’s no past simple to be found. Is this sentence grammatical in the first place? Is there a name for this particular sentence structure?