What is the tyranny Howard Zinn discusses in his chapter "Tyranny is Tyranny."
To understand what tyranny Zinn is talking about, look at the end of the chapter, where Zinn actually uses the phrase "tyranny is tyranny" in a quote. That quote shows that the tyranny he is talking about is the tyranny of the rich and powerful over everyone else.
Zinn argues that the rich and powerful elites in the colonies exploited the poor common people and used them in the rebellion against England. He argues that the rich played on true grievances that the poor without having any intention of improving conditions for the poor. He says that the rich in the colonies had always tyrannized the poor and that, therefore, they were being hypocritical when they complained about British tyranny.
At the end of the chapter, Zinn shows this by pointing out that the rich in Boston were allowed to pay for substitutes instead of having to go into the militia themselves. That caused the common people to say that "tyranny is tyranny" no matter if it was American elites or British elites who were the source of the tyranny.