Personally I feel like it's only the great black one that's a dragon, and the others are wyvern. That could explain the switch when he refers to how the black dragon is crueler, and maybe that's the first they saw in the last chap hence why they said dragons instead of wyverns, even if it was both.
As we don't see the creatures very distinctively, I don't know though.
And it's been long since I was frustrated by a cliffhanger, huh...😛