Looks like a Koi.................
You’re probably right in which case it’s a pretty small Koi. So much for the author’s credibility.
I tend to agree with you. Notice the 2 barbels on either side of the mouth. Koi have those, goldfish don't.
Goldfish and Koi are carp. Colorful carp, but just carp.
Koi have whiskers. That’s a very big regular goldfish.
Super-Koi. They can get pretty big.
The reason they normally don’t grow this big is that they will only grow to their space. So your goldfish will stay small enough to fit within his bowl, but a pond fish will get as big as it can.
Pretty ingenious of God, no?