As @SavoirFaire wrote, it’s not treason.
The word “treason” is often incorrectly used by people who mean someone is betraying their nation.
So for example, if Trump were doing various things to make himself rich rather than serve the nation, at the expense of our environment, civil rights, wasting our money stupidly, paying it to his cronies, appointing incompetent & corrupt people to high government offices to undermine the purposes of government departments, lying to the people all the time, many might consider most/all of those things to be betrayals, and they may not get/care-about the distinction between those types of betrayal and the legal definition of treason.
In this case, some people might very rationally perceive these actions by Kerry as an attempt to preserve peace and diplomacy with Iran, a country which we are not at war with and which many outside the military-industrial complex and those swayed by the stories that we ought to attack Iran, would not like to be at war with Iran.
Such people might, I would think, tend to be glad someone is trying to salvage such agreements, and might sooner feel angered and betrayed that Trump was severing the agreements with Iran, and if that leads to war for corrupt reasons, may feel like that’s a kind of betrayal worth preventing and punishing, whether it’s called treason or not.