From Aliens To Elves

Poul Anderson presents a scientific rationale for why Ythrians, winged extra-solar aliens, can have bodies heavy enough for intelligence and yet be able fly in terrestroid environments and also explains that the elf-like Lunarians are human being genetically modified to live in Lunar gravity although not on the Lunar surface but there is no need to explain the elves and trolls in The Broken Sword because such beings are simply assumed to exist in works of fantasy. However, to our surprise, in his FOREWORD, Anderson suggests that:

magic is control of external phenomena by means not yet known to science but maybe coming under "parapsychology";

an alien metabolism able to live indefinitely, change shape etc might be vulnerable to actinic light or to electrochemical reactions with iron;

such immortals might resort to nonferrous metal and alloys and, e.g., sail like the wind in almost frictionless ships;

they might have built castles earlier than human beings but would not have developed science, e.g., gunpowder or steam engines, in their long-living, aristocratic, conservative, warrior culture.

However, I don't think that any of these rationalizations appear in the text of the novel?