The Gnostic Moses is a whole course, so I'm not sure exactly which lecture in that series you are referring to (or where in the lecture that quote is).
So without knowing precisely what that instructor said, I'll give my take on what (I believe) he meant.
First of all, understand that there is more than one type of Resurrection, and more than one type of immortality.
Most of us are mortal, both in our physical bodies and our internal bodies. But there is a type of immortality that is attained upon engendering the solar bodies.
Whosoever possesses the Astral and Mental bodies becomes absolutely immortal.
- Samael Aun Weor, Alchemy and Kabbalah
But even those who have those solar bodies engendered can still die physically. Their immortality is in the world of the astral and mental planes. But they still have immortality in those planes, and that would be my guess as to what that instructor was referring to.
However, at a much later stage of the path, it is also possible to attain immortality in the physical body, but in order for this to occur, the physical body has to die.
Does that make sense?
As for your second question, previously existing human organisms create new organisms through the process of sex.
But I'm guessing you already knew that.
When he says "These human bodies were developed through the ages," he doesn't mean that the physical bodies you see today are precisely those same bodies that were emitted in an embryonic state at the dawn of life. He means that they
evolved through the ages, and through successive generations, to create the present human bodies.
There is a chapter in
Cosmic Teachings of a Lama called the "The Panspermia of Arrhenius" that explains this process. I also recommend reading the chapter "Evolution and Devolution" in the same book, for further understanding.