Hmmm. I suppose that I can imagine a horse is capable of biting its tongue. I think this is likely of may be an unbroke horse, a very green horse, a horse in a panic, or a horse that falls on its face(don't laugh, it's possible!) But would I expect that a world class dressage horse would bite its tongue for no good reason? Nope, I don't buy that.
Take a look at how Adelinde sometimes rides(as I'm not stalking her, I have no idea how often she does this) her horses. It's called rollkkur and in my opinion *it is my blog* I find it a cruel way to ride for more than a few moments at a time. It looks unnatural, uncomfortable and cruel if the horse is forced to contort itself like that for extended amounts of time, whether the horse is walking or cantering.
I'm impressed with the judges for doing the right thing. Even if the horse accidentally bit its tongue and not because of harm from the rider, they followed the rules. So often in the horse show world, cruelty is ignored for the sake of performance, winning and money.
Here's a couple of pictures of head positions I prefer. Now I'm not saying these photos are perfect, I just prefer the fact that the horses' noses aren't touching their chests.