Trump is running out of diversion material in these days of the Mueller report by citing a Logan Act violation. This conveniently ignores those #TrumpRussia contacts, the 47 GOP senators writing a 2015 love note to Iran, the Trump Inaugural, and then there’s Jared Kushner.
More likely some Trumpian genius decided that since John Kerry is still used in some 2020 polls, that it was worth using that diversion.
In a response to Trump, Kerry’s spokesperson had this to say to CNN:
“Everything President Trump said today is simply wrong, end of story. He’s wrong about the facts, wrong about the law and sadly he’s been wrong about how to use diplomacy to keep America safe. Secretary Kerry helped negotiate a nuclear agreement that helped to solve an intractable problem. The world supported it then, supports it now. We’d hope the president would focus on solving foreign policy problems for America rather than attacking his predecessors for theater.”