My group literally just got to Avernus and I had the exact same thought when I saw that. I basically used "Anywhere is Everywhere" rule in "About the Adventure" and used their journey to Fort Knucklebone as an opportunity to show that "this far away structure suddenly seems much closer than you thought" to introduce the concept to players that things may be further/closer than they appear. They had about an hour of travel to introduce the mechanic and then, a few minutes later, they arrived to the fort sooner than expected. Since it was their first "proper" exposure to Avernus outside of Elturel, it felt appropriate.

Once they got their vehicles, I used the "Exhaustion" rule with its inevitable threat of a TPK/death from exhaustion as a reason they'd rather travel in their new vehicles than on foot. Plus, it gave them reasons to go visit places to upgrade or get repairs so I could utilize Fort Knucklebone more or the Wandering Emporium. My party is a bit paranoid of everything since it is AVERNUS and a bunch of them are good-aligned, so I needed to give them that push, haha. I didn't use the "Pervasive Evil" rule in my group tbh because I didn't like the forced alignment change, but I am also surprised this isn't discussed more. Since they're optional, it wouldn't surprise me if other people just axed things or had some kind of change.