I think the "like wings" to "wings"
can be the exact same thing as the "like wings" to "wings" from Tal-Elmar. And this type of construction is a fairly natural thing in writing, and given that
Tolkien has also written about "Unlight," this can easily be the reading.
In any case, we "know" (externally) post-Moria
Balrogs are "cloaked" in darkness:
" . . . their hearts were of fire, but they were cloaked in darkness, and terror went before them; they had whips of flame. Balrogs they were named." JRRT, The Later
Quenta Silmarillion
Never before (that I recall) in Silmarillion writings were the Balrogs cloaked in shadow or unlight or darkness, but
after JRRT penned the confrontation in
Moria. . . in comes the dark!
I'd also note that pre-Moria Balrogs never (at least certainly) fly anywhere, in battle or in search of anything, and as I noted, in the drafts for the Moria
Balrog encounter, the Balrog is clearly seen -- no shadow no wings -- this is, at the outset, still an "old-concept Balrog" in my opinion.
Let's look at the drafts.
Or not. I'll write out the way I read 'em anyway.
In
draft text A the Balrog is clearly seen and described [no shadow of course].
Gandalf cuts off its hand, a
troll jumps onto the bridge and the Balrog [?tumbled, tumbles] sideways and falls into the chasm, along with Gandalf and the troll.
Draft A text ends, but Tolkien pencils a note:
"Alter description of Balrog. It seemed to be of man's shape, but its form could not be plainly discerned. It felt larger than it looked." Then Tolkien added (after the words
"Through the air it sprang over the fiery fissure")
"and a great shadow seemed to block out the light" -- JRRT also noted that Gandalf should break the bridge, and the
"Balrog falls, but lassoos him."
Tolkien then begins a fair copy of draft A, but decides to do some further draft writing before coming to the end, and then finishes the fair copy (
C), following this new draft (
B).
What did Tolkien change in the new draft about the confrontation with the Balrog? In
B it's said that the Balrog "stood facing him" -- thus the new draft has the shadow of A, but no mention of wings -- and when writing the fair copy C, Tolkien added that the Balrog "halted" facing him
"and the shadow about him reached out like great wings."
So, we basically have three copies where the Balrog has a shadow, and in C, the Shadow about
him [
Gandalf] reaches out like great wings.
Christopher Tolkien notes that neither the second draft B, nor the fair copy C, contain the sentence in which the Balrog drew itself up
"nor speaks of the "wings" (Christopher Tolkien writes "wings" here not simply wings).
Proof? No again.
Of course, the second reference to wings comes in at some point, but does it really seem that JRRT has added both wings and flight, rather than a unique shadow? Not to me. And wings and flight would arguably constitute a notable change with respect to the already-written
battles of the
First Age, wherein
Morgoth would now, early on in the history, have
many, possibly thousands (Balrogs existed in great numbers, at least at the time Tolkien penned the Moria encounter) of flying demons at his disposal.
Maybe Tolkien intended this. . .
. . . but I rather think that Tolkien's concern here was to obscure the creature; indeed
cloak it in shadow as he would describe later in
The Silmarillion (quoted above), cloak it in something like
Ungoliant's Unlight, which was not merely lack of light.
And of course, what an inventive way to get a unique "winged
demon" into the tale! All that's "needed" is the moment where the cloak of darkness looks like (stretching out) wings; and no need to worry about the
tales of the Elder Days, wherein (I think also notably) the winged
dragons are held until the last (Wrathy War).
" . . . for until that day [when Morgoth loosed the winged dragons at the Wrathy War]
no creatures of his cruel thought had yet assailed the air." JRRT Quenta Silmarillion, The Lost Road and Other Writings
To me the idea of fleeing
Thangorodrim is simple.
And we know from the same text you quoted that JRRT has no problem using "flying" for running
orcs. And to quote Tolkien's
Quenta Silmarillion (The Lost Road And Other Writings):
"The Balrogs were destroyed, save some few that fled and hid themselves in caverns inaccessible at the roots of the earth."
I'd flee too! And if flapping my front paws helped. . .