I'm not sure if you noticed yet but they added the Terra Titan to the 2nd titan collection. Here is the image

But notice how it is number 7? well we already have a #7

And here is the Terra titan description screen
Notice how you can get it out of breeding, Breeding sanctuary and a collection? They need to fix this description

Now it shows this

And the description

Notice how the source changed?

I would not take too much stock in that, they did the same thing when they first added the last Titan to the collection/book, This will be updated and changed once he is fully added to the book... if he is breedable (in the beginning) like the Sea Titan the odds of success will be so low that he is practically unbreedable.