Well, I thought it was "single unit" because the fairy alert him about the enemy then right after that, he made an assumption that it was the enemy Flagship.
If it was "What is it exactly", the dialogue wouldn't be that logical I think.
But...What the hell is that thing!?The rain...Is it the Flagship!?Akagi and Hiei are heavily damaged!
And Haruna is moderately damaged!A new model!?That's not it...A single unit!?Everyone! Prepare to battle!Admiral!It's something tremendous...However, this operation...At this moment, I remembered about something.Admiral! There is one source of heat found from the north! It's an abyssal fleet!
Artist typo: 凄艦>棲艦