I didn’t say you CAN’T have threads, I was just saying you don’t need a whole lot of them for a fast rendering engine.
If an app isn’t written in C++ they’re all at the “same disadvantage”. Other than runtime performance, what else could it be? Security (C++ is bad at that)? Reliability (C++ is bad at recovering from errors)? Every language has some sort of flaw.
About it being “best” if someone can’t do better than what’s already there, I agree. Opera Mini on iOS uses its own engine but it’s only on Opera’s proxy servers and not on the actual device but it’s still a browser that doesn’t use the WebKit engine, though.