This is a pretty interesting argument. Basically arguing that, when Eä exists as only the Music of the Ainur, it isn't real at all, and it only becomes real (As in, the actual Eä, the universe) when Eru endows it with the Flame But when Eä becomes real, is precisely when it comes to be on the same level as the Ainur, based on this:
"
Those who became most involved in this work of An, as it was in the first instance, became so engrossed with it, that when the Creator made it real (that is, gave it the secondary reality, subordinate to his own, which we call primary reality, and so in that hierarchy on the same plane with themselves) they desired to enter into it, from the beginning of its 'realization'."
– Letter 200
It does, at prima facie, seem to saying that the Ainur live in the secondary reality, Eru in the primary, and Eä is a story/vision that is given secondary reality on par with the Ainur's by the Flame Imperishable. But Tyranno's already pointed out that the Ainur being of the primary reality is already accepted, and they
are stated to be from "the primary creation," so I'll leave that alone. It's not a topic of discussion for this thread.
So: Let's grant that the Ainur exist in the primary reality, and that when the Ainulindale is given existence by Eru, it becomes a secondary reality relative to themselves. Looking at it now, I think a problem for them being High 1-A+ would come into play
here: It's been clarified that, as far as can be ascertained, knowledge is the definer of power for the Ainur. Yet, if their knowledge is their power at that level, them being High 1-A+ would pretty much necessitate that their knowledge exhausts everything that Eru can create, which doesn't seem correct.
For that matter, the Star Maker was brought up as an example of a character who has the raw capacity but not the knowledge to make all possible worlds, but that case is different precisely because, in there, power and knowledge are distinct. Here, they're being argued to be identical. So there's not much of a comparison.
Otherwise: The reason for why the statement about writers having no limits save the laws of contradiction is applied to the Ainur is that the Ainur are from "The Primary Reality," which is also what Tolkien terms the real world in which writers like himself live. Now, Tolkien and etc. don't actually exist in the Legendarium, so the argument seems to be moreso that Tolkien considers the Ainur to be functionally of the same nature as writers are in his understanding of the world, and as such the statements about the latter are applicable to the former.
Compounded with the issue of the Ainur's knowledge, I'm admittedly not as sold on this line of reasoning. Mostly because it's missing a link: There is a gap between "The Ainur live in a primary reality to which Eä is secondaries" and "Therefore they have the same abilities as writers." This gap as it stands seems to be filled with "Tolkien uses the term "primary reality" to talk about both where the Ainur are from and the real world," but I do struggle to see how that's adequate justification at all. In Eru's case, at least it's talking about the exact same thing (i.e. God considered as the wellspring of all possible stories and worlds), which isn't the case with the Ainur/Writers comparison.
So, I'm neutral on High 1-A+ Ainur as it stands, since I'd like more information to collapse my stance towards agreeing or disagreeing.
(This is only for the Ainur, by the way. Obviously, I think Eru is still perfectly fine at 0)