That’s just a variant of the ages old Philosophy question “What is real?”
Last I checked the best answer there was is “I think therefore I am” (Descartes), which is quite old and doesn’t even deal with the whole “what am I”, much less with the existance or not of everything else.
“Is the Internet all AI but me” is actually pretty mild skepticism in this domain - I mean, how sure are your that your’re not some kind of advanced AI yourself who believes itself to be “human” or even that the whole “human” concept is at all real and not just part of an advanced universe simulation with “generative simulated organic life” inv which various AIs which are unaware of their AI status, such as yourself, participate?
Or maybe you’re just one of the brains of a 5-dimensional hyper intelligence and “life as a human” is but a game they play for such minor brains to keep them occupied…