That's not to do with Dark Forest Theory, but why do you say it's about how fast information travels? I take it as a theory about the outlook of technological civilisations with respect to each other.

And yes, lifespans are important. Perhaps not in the books, but in the real world our science and technology advances at least some in every generation, as new scientists and technologists continue the work of their forebears.

If human generations lasted a thousand years, it would perhaps take us a lot longer to make the same technological progress that it now only takes us a few years- if nothing else because the urgency of making progress would be reduced accordingly.

Lifespans may also influence the actualy speed by which species form thoughts and communicate. Remember the Ents in The Lord of the Rings? A species that lived ten thousand years may take an Earth day to form a meaningful utterance. Technological advancement would take considerably longer for them than for us.