Awww, and I was all set, combing through Morrowind's editor with several quotes detailing the Argonian's life-springing relationship with the Hist.
Oddly, in The Annotated Annuad the Ehlnofey are described as the ancestors of Mer and Men, while the Hist are simply described as the "trees of Argonia." Considering the context of the paragraph, if the Hist in fact actually created the Argonians, it would have made sense to include that information in the text. In fact, there's no mention of the Argonians at all, perhaps a meaningful omission.
To anyone from an actual source of civilization that is capable of scholarly discussion, Black Marsh and the Argonians have always been a. Mysterious, b. uninteresting for whatever reason, c. inaccessable and very tight-lipped about said mysteries, d. lesser/a beast race and therefore less than Man or Mer. Therefore, we only have off-hand, in-games quotes, usually by Argonians, a few books, and the fighter's guild quest-line.
The last source is our only good source of information on Hist right now, which confirmed that the Hist interact with the world around it by secreting magickal/alchemical sap or secretions that is lapped up by the environment and creatures around it, effecting change upon its surroundings. Since one Hist could make such a specific secretion with such a specific effect on creatures it likely had never encountered before, I'd imagine that multiple Hist working in concert with each other could change a species over time.
If it makes any difference, somewhere in the back of my skull I remember the prevailing idea was that the Hist coopted a species living in Black Marsh, changed them to be Argonians, and gave them sentience. So, made them, yes, but created, especially in the ES world, implies that one instant they weren't, and then the next they were.