Why Isildur
Isildur and Thomas Avasol are the same person. Thomas Avasol builds Aedelgard through Millenion AB; online, and in the Aedelgard community, he goes by Isildur.
Not the Isildur people remember
Most people remember Isildur as the man who cut the Ring from Sauron's hand and would not give it up. That is not the one this name is for.
Long before that, in Tolkien's account of the fall of Númenor, the king was being turned against everything old, and the White Tree, Nimloth, was marked for burning. Isildur went alone and in disguise into the king's court, took a single fruit from the tree, and fought his way out. He was so badly wounded that he nearly died. The tree was cut down and burned. The fruit survived, and from it the line of the White Tree continued, long after the kingdom that burned it was gone.
Why that matters here
That one act is Aedelgard. Platforms rise and fall, and models are replaced every few months. What should outlast them is what you have grown: your memory, your context, the mind you have built up over time. It should never be locked inside something that can burn.
So Aedelgard keeps the mind separate from the model that thinks with it, keeps it portable, and helps you take it with you, including away from us. Carry the seed out of the fire, and the line continues.