It was made as response to this comment, from Munin:
But the author could be any other Shardholder, really. Or anyone else with the ability to travel between worlds.
Oh, I'm firmly of the belief that the person writing the letter is Hoid. That interlude is the best evidence around, although there is plenty more. You're overstating your case, Munin.
The person writing the letter has the following attributes:
(1) Knew the Shardbearers personally before they bore the shards. This is supported by the references to Ati and Rayse.
(2) Has observed the events on multiple worlds. References to Sel and Ati make this clear.
(3) Has a quest of some sort, some goal he is trying to reach, which involves personal motivations against Rayse and Bavadin.
(4) Is being chased by the 17th shard, but has successfully laid down a false trail for them.
(5) Seriously needs help.
(6) Is directly involved in the events of the war that is presumably coming.
Hoid satisfies all of these conditions easily. This is more rigid than you might think; Sazed, for instance, is ruled out by condition (1). Most entities who would satisfy conditions 1-3 and 5-6 would have to be somebody new, somebody we have never yet seen. This isn't impossible, but we just don't know enough to propose anything. Condition (5) does suggest that the author is not uberpowerful, but given what Odium has done this probably doesn't mean much.
By contrast, condition 4 is ridiculously specific, and exactly what we see in the interlude under discussion, where the searchers look for Hoid by name. This point alone suggests Hoid as the author, barring really strong evidence otherwise.
Also, Hoid has been seen to directly interfere on Shardworlds, whereas we have no canonical evidence of other interference, so condition 6 suggests Hoid much more strongly than it does any other entity. Condition 3 would go a long way to explaining Hoid's behavior in other books.
In addition, just how many planethoppers are there floating around? All we've seen is Hoid and the 17th shard. The reference pool really is quite small here.
Thoughts? I think the logic is fairly sound.