The idea that "myth" means "something that isn't true" is a relatively recent accretion (19th century). Originally it simply referred to a legendary tale, one primarily concerned with the gods and their dealings with humanity. In the ancient world, myths were also considered "true," but there were different types of truth. What we think of as truth today is an empirical truth--a truth that can be argued, tested, and proven. Myths, however, spoke to inherent truths that transcended empiricism. I believe that this is what Velorien means by the possibility that myths can still be "true" even if they are not empirically true. That is, they speak to truths about the human nature and the human condition.
There is also a long academic tradition of associating language with creation. It is no accident that in the Christian Bible, for example, God speaks the world into existence. To put it in very rough and simple terms, language is a symbolic system devised by humanity not only as a means of facilitating communication, but as a very way of thinking. Thus language is essential to our perception of the world. (This can be best seen in the different ways that different languages classify and categorize reality. To give a very simple example, the perception of the colors blue and green differs in English and Korean (and Japanese, for that matter). What in English we call a green light is referred to in Korean as a blue light--but the actual color is the same.) Most people think of "reality" as existing as some abstract thing, but the truth is that, as far as humans are concerned, reality doesn't exist outside of our perception of it. Or, to put it in less solipsistic terms, one person's reality may be very different from another person's reality. That being the case, since perception of reality is in one sense also creation of reality, language is capable of creation as well. A lot of primitive magic, for example, revolved around the principle that you could make something true just by affirming it aloud. In fact, people still believe this today, to some extent. Any time someone tells you, "Don't say such horrible things!" they are following this principle. By saying something horrible, you are introducing the idea and making it a part of that person's reality. (There is even a Korean phrase that says, "words become seeds," meaning that just saying something can make it come true.)
Hmm. I guess that wasn't that "rough and simple" after all, was it? Sorry about that. I got a little carried away. I can only hope that someone finds this mildly interesting.
Actually, it's the other way around: Velorien is using "myth" in the original, stricter sense of the term. You are using the more modern, looser sense. To put it another way, Velorien is using the term in its literary or folkloric sense, while you are using it in its general sense.