I think that the real question is what is the dividing line between "TauPhies-like creatures" in the first paragraph and the examples in the second. What is it about the examples in the second paragraph that justifies labeling them as "dreams" versus simply "Tau-Phi-like creatures which are 10% stronger or 10% longer-lived" than the example we have here and now?
The dividing line is our understanding of nature based on our experiences and acquired knowledge. People who get confident about reality based on what they can imagine are prone to venture boldly into the realms which may or may not exist. Problems start when they conflate reality with their imagination.
10% longer-lived? Sure, we know that variation in human lifespan can be higher than 10%. Reasonable possibility.
1000 year old? We never observed living humans even close to that. We currently don't know a way to extend our life by that much. Can we imagine people living that long? Sure. We can imagine innumerable creatures having lifespans from 0 to infinity. Do they exist? We don't know but it's not reasonable to claim that they do only because we can imagine them, is it? This is what Velleius did in his description of isonomia.