And were instantly recognized by the crew of the Kelvin as Romulans, even before the alleged changes to the timeline were made.
I don't think they were "instantly recognized" by the Kelvin crew. There's no dialogue in the film to support that contention. Someone figured out after the fact that they were Romulans, but that could have been because (a) the Kelvin survivors were able to download info from the Narada, or (b) they heard over the coms that the Klingons captured some Romulans in a huge ship right after the Kelvin was attacked.