On Twitter, Bret Weinstein has floated the hypothesis that the "Biden absence" is a Honeypot for conspiracy theorist. Like the forged poor military records of George W Bush that got Dan Rather fired (but were probably real).

His hypothesis is basically that enough red meat (no letterhead, weird signature, Kamala's awkward performances, zero public appearances, etc) was dangled so that those who question narratives can come out of the woodwork. Then, Biden reappears, quite "healthy" (relative to Biden's baseline), and those who questioned the weird hints look like fools.