Yes, a simple out of control campfire can burn a lot of forest. Of course, it needs the proper wood to start. If you let a small fire and put a healthy branch/tree on it, you'll notice it doesn't burn as easily. You need some dead wood to first start the fire and make it big enough to sustain itself and affect the healthy wood.
And now, there's only one problem. Teldrassil was a living tree, protected and cleaned by its denizens and whose health was ensured by druids. So, no dead wood to start. What those flaming boulders did is the equivalent of you holding a lighter to a living tree for a very short time. Chances are it won't burn.
But wait, there's more! Teldrassil isn't just a living tree. It's one that also received blessings of protection and, furthermore, sits in the middle of the ocean. It takes just a few night elves to take water and put out any fire starting because of that.
And wait, there's even more, you see, a few expansions ago there was this random dragon that passed right by breathing fire. It destroyed Auberdine yet the fire of Deathwing didn't even light a spark in Teldrassil when it had not been blessed and it was corrupted and infested by the Nightmare?
No, actually, forget Deathwing. There were satyrs and imps on Teldrassil and they were loyal to the Burning Legion. Are you telling me there isn't a single one that thought to light it on fire?
Magic wood is more flammable? Gee, there's a footman with a torch near the Teldrassil bank, I wonder how he didn't burn it for so long then!
Come one, magic is supposed to imbue the object/plant/animal/humanoid, not make it weak. Curses are for that.
The distance from Darkshore to Teldrassil is far enough. If you look at the map, the distance is somewhat similar to the one between Gotland and Latvia, Cuba and Florida or Taiwan and China. Saying that it's "a bit unrealistic" for a catapult is putting it mildly. It's impossible for a catapult. We're talking distances of at least 120km/100miles. Even with magic, that breaks any reasonable belief of reality.