TPM is not a prequel to A New Hope...
Yes, it is. It's just not the only one.
That's why it's been called a "Star Wars prequel" from day one and no one ever disputed that. Similarly, the PT is referred to as "the prequels" as opposed to "one prequel and a prequel to that prequel and a prequel to that prequel to the prequel".
If you try to change the definition of prequel so that TPM is not a prequel to ANH, then what exactly is it a prequel to? AOTC? That would mean it's a prequel to a film which came out three years after its own release. Which kind of invalidates the "quel" part.