The Treaty of Tordesillas allowed Portugal to claim the entirety of Brazil when they only had a few cities along the coastline, for instance. Similarly, Tin's logic is, "We claimed those islands, we started colonizing them, and then Yllendthyr swooped in and grabbed part before we could get there!"

Tin is like a North Korea that actually has enough power to be taken seriously. Nobody is going to stop Babur from sending a letter.

It will probably have the effect of making Yllendthyr fortify, yes, but Tin is about to do the same thing. When the elves see this they'd sen in more troops letter or not.

Keep in mind that Babur also blames the elves for arming the natives and inciting rebellion. He feels compelled to get them out, and soon.