As a broadcast engineer, I can tell you for certain 1) that it is not flat; and 2) it hews closely to sphericity. If it were not so, the equations we use for determining radio field strength would not work.
Discussion
As a shaman who travels the multiverse, I can tell you for certain it is all an illusion of the mind, a story. All we perceive to be true is true within our own story or timeline. Or not. It is, after all, April fools. 🤷