Lighter-than-air vehicles were already existing and anyone could see that birds provided a heavier-than-air example of locomotion.

The question was more about whether heavier than air vehicles could ever lift humans in the air. Lord Kelvin, particularly, considered it impossible because he made the assumption that such a vehicle would need to be human-powered. He argued that the kind of wing-flipping that allowed birds to fly was impossible with a human musculature (which has even recently been demonstrated as false)

About the state of sci-fi, I know of at least one 1886 book by Jules Verne: Robur le Conquérant.

https://en.wikipedia.org/wiki/Robur_the_Conqueror

It talks about this guy who built an electric-powered helicopter and traveled around the world with it.

Now the thing that many (all?) SF author had missed, prior the Wright brothers, was that lift would not be given by propellers pointing up. Lifting a mass through propellers is much less efficient than using the lift of a wing.