Well, to address a few points.
What is the level that the Truths work on?
Outerverse level+, on all layers. Since they work the same on all of them.
Why?
It directly erases a person to the level of the Sea of Oblivion, a 1-A+ realm. This means it's potency is within that level as one can't erase to such a level without having a similar potency. It doesn't need to specifically affect a 1-A+ character to cause the erasure.
Why is the Sea of Oblivion 1-A+?
Because it encompasses the entirety of both the Witch Domain and the Third Domain. The Third Domain is a structure that views the Witch Domain like the Witch Domain views the Human Domain, and contains infinite layers like the Witch Domain does itself. The Sea of Oblivion encompasses it because it is the grounds where even Featherine goes when she stops thinking, a layer of nonexistence until she thinks again.
Why is this not a consequence of them dying and instead a direct effect of the Truths themselves?
There are two ways to 'die' in the Meta-World, as you can't die from just being killed to death. One, you stop thinking, and two, you get erased by a Truth.
Upon being hit by a Truth, you will be sent into the Sea of Oblivion until you can finally think of a proper counter to the Truth (like Erika did). This obviously only applies in-verse as it gives all characters High-Godly regeneration. This is why Truths are not the former, as it doesn't prevent meta-beings from thinking and forcibly erases them anyways.
If they were to die in the only other ways in the Meta-World, (i.e. they stop thinking or are forcibly taken off the game board by a higher being) they wouldn't be able to actively try and 'think' while in nonexistence.
