Actual estimate: 33-50%

This comes from a verbal interview (from the 14:14 mark). The interview was focused on AI, and this estimate may have been as well. Tallinn said he's not very confident, but is fairly confident his estimate would be in double-digits, and then said "two obvious Schelling points" are 33% or 50%, so he'd guess somewhere in between those. Other comments during the interview seem to imply Tallinn is either just talking about extinction risk or thinks existential risk happens to be dominated by extinction risk.