Elsewhere in this thread he links to David Teter's OPEN-RISOP as his source. [1]
The quality of OPEN-RISOP is generally pretty good, and the author worked at Sandia National Labs about 10-20 years ago. Targeting at least is pretty consistent with what I'd expect as high value targets. OPEN-RISOP's counterforce + countervalue scenario only tallies about 60M fatalities (~20% of US population) from the original attack. chakalakaps must be assuming cascading failures of infrastructure & food production after the initial attack (a la Threads [2]). While this is a distinct possibility, it's far from a given - and this is also one of those areas where being prepared (eg. having a year of dried foodstuffs stored, or a small personal garden) can give you a distinct survival advantage.
David has mentioned on Twitter and Reddit (when asked) that he estimates 90% to 95% of all people living in the U.S. would be dead within 18 months after either of his attack scenarios that involve countervalue due to complete breakdown of all infrastructure, production, supply chains, communications, etc.