Even more supportive of your argument is the original text from which that passage in the '77 was adapted:
Quote:
|
Far beneath the halls of Angband, in vaults to which the Valar in the haste of their assault had not descended, the Balrogs lurked still, awaiting ever the return of their lord. Swiftly they arose, and they passed with winged speed over Hithlum, and they came to Lammoth as a tempest of fire.
|
The Later Quenta Silmarillion Part One, HoMe X
According to me, it's possible that the Balrogs were not yet incarnated at this time, which leaves a number of possible explanations.
See also
this thread for more along burra's line of reasoning.