> The placement of the tongue is exactly between those two letters.
That is not the origin of the digraph "th" (and also I would suggest picking up an introductory textbook of phonetics, because you’re way off). The digraph comes from the fact that in Classical Greek the unvoiced dental stop denoted by the letter theta was pronounced with aspiration, but in Latin the native dental stop denoted by the letter T was pronounced without aspiration. Therefore, when the Romans had to represent Greek words with theta in their own alphabet, they added an H after the T to mark the aspiration.
That is not the origin of the digraph "th" (and also I would suggest picking up an introductory textbook of phonetics, because you’re way off). The digraph comes from the fact that in Classical Greek the unvoiced dental stop denoted by the letter theta was pronounced with aspiration, but in Latin the native dental stop denoted by the letter T was pronounced without aspiration. Therefore, when the Romans had to represent Greek words with theta in their own alphabet, they added an H after the T to mark the aspiration.