The C&Ds on Twitter offer no rationale for the takedown request (i.e. DMCA claims or whatever). It looks like they're just "asking nicely" at this point.
I'm still waiting for someone to send a take-down notice to ToS's on major platforms. That'll be entertaining.
That's precisely what a cease and desist is: asking nicely, possibly with legal merit, possibly not.
I suppose at the end of the day, everyone receiving nastygrams could have a contract with AWS that prohibits disclosing the names of talks at their conference. That is a shitty condition to attach to a cloud service and should not be legal, but it probably is. (I wonder what speech conditions I agree to for receiving electricity at my house. I've never checked.)