>In an interview with Robert Wright in 2003, Dyson referred to his paper on the search for Dyson spheres as "a little joke" and commented that "you get to be famous only for the things you don't think are serious" [...]

To be fair, he later added this:

>in a later interview with students from The University of Edinburgh in 2018, he referred to the premise of the Dyson sphere as being "correct and uncontroversial".[13] In other interviews, while lamenting the naming of the object, Dyson commented that "the idea was a good one", and referred to his contribution to a paper on disassembling planets as a means of constructing one.

Sources are in: https://en.wikipedia.org/wiki/Dyson_sphere

Have you read the paper itself, not just summaries of the idea? It's obvious from the way he wrote it, dripping in sarcasm. Talking about "Malthusian principles" and "Lebensraum", while hand waving away any common sense questions about how the mass of Jupiter would even be smeared into a sphere around the sun, just saying that he can conceive of it and therefore we should spend public money looking for it. He's having a lark.

Also, he literally said it was a joke, and was miffed that he was best know for something he didn't take seriously.

https://www.youtube.com/watch?v=fLzEX1TPBFM