Short answer: Trust the guide. It’s based on many years of practice and experience. The recommended lvl’s are correct as the zones are designed to be done in several stages. Example. The last part of Redridge has 27-28 mobs. One of the first quests is a lvl 15 pickup “Redridge Goulash”. The pigs are around lvl 18. They charge and they hurt. Would it make sense doing Redridge before lvl 18? Nope, Loch Modan as an example has easy fast completed quests and hardly any competition. It’s all about efficiency and being able to solo and “speed level”.
Long answer: Westfall is terrible for leveling. It’s possible but it will be time consuming and frustrating for a lvl 12. It’ll be a lot of grinding and chasing pigs, wolfs & condor’s all over the map, where you’ll run into orange and red mobs. Defias and especially Harvesters will eat you alive. Immune, non tauntable & nets, ranged that hurt .. and Murlocs. These run in packs and you’re going to have to ride the edge and pick them one by one and wait for spawns. Level 19 Dust Devils says hello. They are the Fel Reavers of Westfall and always seem to come when you least expect them. There’s a reason speed levelers always avoid this zone 
You’re eventually going to have to travel to Kalimdor and you’re going to need the Flightpaths. The guide knows this and later it also knows the shortcuts so you don’t have to do the long run. So when it directs you to a weird path through the mountains and tells you to jump down and die, It all makes sense when you spawn at the location and you’ve just saved 30 minutes of running.
You’re even told to do some quests while you run, because “always be questing”. It makes sense. Later you’ll thank the guide for making it easy to travel to Kalimdor, while your friends sit there and moan that they’re running out of stuff to do in their zone because it’s too hard 
Have I done Westfall in the past? Sure. I’ve done all the zones. I know what to avoid like the plaque 