But they had had visual contact with the crew of the Narada, so that's what changed...
Problem is... nowhere is it mentioned that these are Romulans that attacked the Kelvin.
They only had the readings from their sensors to identify who was attacking them.
So this attack could've been carried out by renegade Vulcans, a Vulcan-oid species or possibly the Rigelians who have a similar make-up.
Journey to Babel wrote:
SPOCK: Rigelian physiology is very similar to Vulcan
Plus the attack took place near the Klingon border.